maildrop uses variables to access and manipulate messages. Variables are arbitrary text accessed by referring to the name of the variable, such as HOME, or DEFAULT. Text is placed into a variable by using an assignment statement, such as:

FILE="IN.junk"

This statement puts the text "IN.junk" (without the quotes) into a variable whose name is FILE. Later, the contents of a variable are accessed by using the $ symbol and the name for the variable. For example:

to $FILE

This will deliver the current message to the mailbox file (or a maildir directory) named "IN.junk".

maildrop initially creates variables from the environment variables of the operating system, UNLESS maildrop runs in delivery mode. Each operating system environment variable becomes a maildrop variable. When running in delivery mode, maildrop does not import the environment for security reasons. In all cases maildrop resets the following variables to their default values: HOME, DEFAULT, SHELL, PATH, LOCKEXT, LOCKREFRESH, LOCKSLEEP, LOCKTIMEOUT, MAILDIRQUOTA, SENDMAIL and LOGNAME.

There's one exception to this rule which applies to the version of maildrop that comes with the Courier Mail Server. The following does not apply to the standalone version of maildrop: when running in delivery mode, if the -d flag was not used, or if it specifies the same userid as the one that's running maildrop, the following variables are automatically imported from the environment: HOME, SHELL, LOGNAME and MAILDIRQUOTA. These environment variables are initialized by Courier prior to running maildrop. Additionally, the initial value for the DEFAULT maildrop variable is imported from the MAILDROPDEFAULT environment variable. This is because Courier overloads the DEFAULT environment variable to store the defaulted portion of the local mailbox address. See the dot-courier(5) man page in the Courier distribution. You can grab Courier's DEFAULT value by using the import command. Note, however, that this will clobber the old contents of DEFAULT, which is probably not what you want. The right way to do this would be something like this:

SAVEDEFAULT=$DEFAULT
import DEFAULT
LOCALDEFAULT=$DEFAULT
DEFAULT=$SAVEDEFAULT

All internal variables are exported back as environment variables when maildrop runs an external command. Changes to internal variables, made by the filter file, are reflected in the exported environment.

Most whitespace is generally ignored. The # character introduces a comment running to the end of the line, which is also ignored. Unlike other mail filters, maildrop parses the filter file before taking any action with the message. If there are syntax errors in the file, maildrop displays an error message, and returns EX_TEMPFAIL. That should cause the mail message to remain in the queue, and, hopefully allow the problem to be corrected, without bouncing any mail. Note: In maildrop, the end of line is a lexical token. In order to continue a long statement on the next line, terminate the line with a backslash character.

Literal text in the maildrop filtering language is surrounded by either single or double quotes. In order to enter a single quote into a text literal surrounded by single quotes, or a double quote into a literal surrounded by double quotes, prefix it with a backslash character. Use two backslash characters characters to enter one backslash character in the text literal. Note: A backslash followed by either a backslash, or a matching quote, is the only situation where the backslash character is actually removed, leaving only the following character in the actual text literal. If a backslash character is followed by any other character, the backslash is NOT removed.

Multiple text literals in a row are automatically concatenated, even if they use different quotes. For example:

FOOBAR="Foo"'bar'
SAVEDEFAULT=$DEFAULT
import DEFAULT
LOCALDEFAULT=$DEFAULT
DEFAULT=$SAVEDEFAULT

This sets the variable FOOBAR to the text "Foobar".

Variable substitution is performed on text literals that's surrounded by double quotation marks. The "$" character, followed by a variable name, is replaced by that variable's contents.

MAILBOX="$HOME/Mailbox"

This sets the variable MAILBOX to the contents of the variable HOME followed by "/Mailbox". Variable names must begin with an uppercase letter, a lowercase letter, or an underscore. Following that, all letters, digits, and underscores are taken as a variable name, and its contents replace the $ sign, and the variable name. It is possible to access variables whose name includes other characters, by using braces as follows:

MAILBOX="${HOME-WORD}/Mailbox"

Inserts the contents of the HOME-WORD variable. If the variable does not exist, the empty text literal is used to replace the variable name. It is not possible to access variables whose names include the } character.

If the $ character is not followed by a left brace, letter, or an underscore, the $ character remains unmolested in the text literal. A backslash followed by the $ character results in a $ character in the text literal, without doing any variable substitution.

Variable substitution is not done in text literals which are surrounded by single quotes (apostrophes).

maildrop initializes special variables: $1, $2, and so on, with additional parameters specified on the maildrop command line. A filter file may use those variables just like any other variables.

The following variables are automatically defined by maildrop. The default values for the following variables may be changed by the system administrator. For security reasons, the values of the following variables are always reset to their default values, and are never imported from the environment:

DEFAULT

The default mailbox to deliver the message to. If the filter file does not indicate a mailbox to deliver this message to, the message is delivered to this mailbox. The default mailbox is defined by the system administrator.

FROM

Message envelope sender. This is usually the same address as what appears in the From: header, but may not be. This information may or may not be available to maildrop on your system. The message envelope sender is usually specified with the -f option to maildrop. If the -f option is not given, maildrop looks for the From_ line in the message. As the last resort, FROM defaults to the userid which invoked maildrop. Note that FROM may be empty - the message envelope sender is empty for bounce messages.

HOME

Home directory of the user running maildrop.

HOSTNAME

Network name of the machine running maildrop. Obtained from gethostname(3).

LOCKEXT

Extension for dot-lock files (default: .lock).

LOCKREFRESH

Refresh interval, in seconds, for dot-locks (default: 15). When maildrop dot-locks a mailbox, maildrop tries to refresh the lock periodically in order to keep other programs from removing a stale dot-lock. This is only required if a dot-lock exists for a prolonged period of time, which should be discouraged anyway.

LOCKSLEEP

Number of seconds to wait to try again to create a dot-lock file, if one already exists (default: 5).

LOCKTIMEOUT

Number of seconds to wait before removing a stale dot-lock file (default: 60). If a dot-lock file still exists after LOCKTIMEOUT seconds, maildrop assumes that the process holding the lock no longer exists, and the dot-lock file can be safely removed. After removing the dot-lock file, maildrop waits LOCKSLEEP seconds before trying to create its own dot-lock file, in order to avoid a race condition with another process which is also trying to remove the same stale dot-lock, at the same time.

LOGNAME

Name of the user to who the message is being delivered.

MAILFILTER

This is the name of the original filter file that was given to maildrop on the command line. This is mostly usefull to -default filter files, it allows them to obtain the value of the -M option specified on the command line.

PATH

Command execution path. maildrop resets PATH to the system default (usually /bin:/usr/bin:/usr/local/bin).

SENDMAIL

The mail delivery agent. When maildrop is instructed to deliver the message to a mailbox whose name begins with the ! character, this is interpreted as a request to forward the message. The SENDMAIL command is executed to forward the message.

SHELL

The login shell. The shell is used to execute all commands invoked by maildrop.

VERBOSE

Current Debug level (default: 0). Setting VERBOSE to progressive higher values, between 1 and 9, produces debugging output on standard error. maildrop ignores the VERBOSE variable in delivery mode (in order not to confuse the mail transport agent).

UMASK

The file creation mode mask, in octal. The default setting of 077 creates mailboxes that are readable and writable by the owner only. Use 007 to create mailboxes that are readable/writable by both owner and the group. Use 037 to create mailboxes that are readable by both owner and group, but writable by owner only. Permissions on existing mailboxes are not changed, this setting affects only new mailboxes. When delivering to maildirs this setting sets the permissions on new messages only. Access permissions on messages in maildirs are also affected by the permissions on the maildir directories.

The following variables are automatically used by maildrop when the filter file is being processed:

EXITCODE

Return code for maildrop. When maildrop successfully delivers a message, it terminates with this exit code, which defaults to 0. When the to or the cc command is used to deliver the message to an external process, via a pipe, maildrop will set this variable to the exit code of the external process. Since maildrop immediately terminates after completing the to command this means that maildrop's exit code will be the exit code of the external process. If the to command does not deliver the message to a process you must set EXITCODE before the to command, since maildrop terminates immediately after finishing the delivery.

LINES

Number of lines in the current message. Note that this may be an approximation. It may or may not take into account the -A option, or any mbox "From_" lines. Use this as criteria for filtering, nothing more.

MAILDIRQUOTA

Set this variable in order to manually enforce a maximum size on ANY maildir where the message is delivered. This is an optional feature that must be enabled by the system administrator, see maildirquota(8) for more information.

RETURNCODE

This variable is set when maildrop runs the xfilter command, or a command that's specified within a pair of backtick characters ( command substitution ). The RETURNCODE variable will be set to the exit code of the command, after it completes.

SIZE

Number of bytes in the message. This may or may not include the -A option, and the mbox From_ line. Use this as a criteria for filtering, nothing more.

All text strings in filter files should be in single, or double quotes. However, for convenience sake, quotes can be omitted under certain circumstances.

Text that includes ONLY letters, digits, and the following characters: _-.:/${}@ may appear without quotes. Note that this does not allow spaces, or backslashes to be entered, however the text is still variable-substituted, and the substituted text may contain other characters.

Also, note that patterns (see below) begin with the slash character. Normally, anything that begins with the slash is interpreted as a pattern. However, text immediately after ``VARIABLE='' is interpreted as a string even if it begins with a slash. This is why something like:

MAILDIR=/var/mail

works as expected. Using quotes, though, is highly recommended. You must use quotes to set a variable to a lone slash, because an unquoted slash is interpreted as a division sign.

Text enclosed in back-tick characters is interpreted as a shell command. The shell command is executed as a child process by maildrop. Its output is used in place of the command. For example:

DIR=`ls`

places the names of the files in the current directory into the DIR variable.

The output of the command will have all newline characters replaced by spaces, and leading and trailing spaces will be stripped (multiple spaces are not removed, though). Also, the contents of the message being delivered is made available to the command on standard input.

The pattern syntax in maildrop is similar to the grep command's syntax, with some minor differences. A pattern takes the following form in the filter file:

/pattern/:options

pattern specifies the text to look for in the message. pattern may not start with a space, because the leading slash will then be interpreted as a division sign. If you must search for text that starts with a space, use something like "/[ ] ... /".

With the exception of the following characters, an exact match is looked for. The following special characters are used to specify complex patterns. If it is necessary to look for one of the following characters, verbatim, in the message, put a backslash in front of it. "x" and "abc" designates an arbitrary character, or a pattern, which may include other special characters as well.

x*

Look for zero, or more, occurrences of the single character x.

(abc)*

Look for zero or more occurrences of abc.

x+

Look for at least one, may be more, occurrences of the single character x.

(abc)+

Look for at least one, or more, occurrences of abc.

x?

Look for zero, or one occurrence of the single character x.

.

The period matches any character except the newline character. (This is also applicable within sets - see below).

(abc)?

look for zero, or one occurrence, of "abc".

abc!def

The exclamation mark is used to separate sections of matched pattern. See "Pattern Match Results" below.

[abc]

Specifies a set of characters - this matches one character, either a, b, or c. List all characters that can be matched between the brackets. One or more characters can be listed. For example, [~=+] matches either a tilde, an equals sign, or a +. This is EXACTLY equivalent to (~|=|+), however with large list of characters this notation is much shorter. In addition, [a-b] matches any character between a and b, inclusive. For example: [0-9] matches a digit. [0-9A-Za-z_] matches either a digit, a letter, or an underscore. To include the dash, the left or the right bracket character in the set itself, prefix them with a backslash. Use two backslashes to include the backslash character itself. (Other, historical ways of including these special characters are permitted, but discouraged).

[^abc]

specifies a set of characters that is NOT the given characters indicated. If the set begins with the ^ symbol, it matches a single character that is NOT any of the characters listed.

\x

where "x" is a character. Specifies that x must be matched exactly. In order to match any special character verbatim (for example - you want to match the asterisk, *) you must prefix it with a backslash character. Use two backslashes in order to match a backslash character verbatim. Also, there are several characters reserved for matching control characters, without having to enter them verbatim into the pattern. See below for more information.

\nnn

specifies a character in octal. "nnn" is an octal number.

The following shorthand notation can be used to specify some common sets. Note that maildrop always uses the "C" locale:

[:alnum:]

An alphanumeric character, same as [0-9A-Za-z].

[:alpha:]

An uppercase or a lowercase letter, same as [A-Za-z].

[:cntrl:]

A control character.

[:digit:]

A digit, same as [0-9].

[:graph:]

A "graphable" character (a non-control character that's not a space).

[:lower:]

A lowercase letter, same as[a-z].

[:print:]

A printable character (not a control character).

[:punct:]

A punctuation character.

[:space:]

Any whitespace character.

[:upper:]

An uppercase letter, same as [A-Z].

[:wbreak:]

Any character other than a letter, digit, or an underscore, same as [^a-zA-Z0-9_].

[:xdigit:]

A hexadecimal digit, same as [0-9A-Fa-f].

To match any special character, put a backslash in front of it. For example, \? matches the question mark. To match a backslash, use \\.

Normally, the pattern can be found anywhere within the message header or body (see below). However, putting the character ^ at the beginning of the pattern forces the pattern to be matched against the beginning of the line only. Putting the character $ at the end of the pattern forces the pattern to be matched against the end of the line only.

Elsewhere in the pattern, the $ sign is used for variable substitution (see above). To include the $ character in the pattern, prefix it with a backslash.

The following backslashed characters are interpreted as follows, when used in a pattern specification:

\n

Matches a newline character. Note - this is only effective if the w option is specified, because without this option maildrop does not recognize multi-line patterns.

\r

Matches a carriage return. Note - maildrop automatically ignores carriage returns at the end of each line. I don't know why you'd want to use \r, but it's there if you need it.

\t

Matches a tab.

\f

Matches a form feed.

\v

Matches a vertical tab.

Long double or singly-quoted text can be broken across multiple lines by ending the line with a lone backslash character, like this:

TEXT="This is a long \
 text string"

The backslash, the newline, and all leading whitespace on the next line is removed, resulting in "This is a long text string".

Following /pattern/, there may be an optional colon, followed by one. or more options. The following options may be specified in any order:

h

Match this pattern against the message header.

b

Match this pattern against the message body.

w

Match this pattern against the entire message and/or body.

D

This is a case sensitive match. Normally the patterns match either uppercase or lowercase text. /john/ will match "John", "john", or "JOHN". Specify the D option for a case-sensitive search: lowercase letters in the pattern must match lowercase letters in the message; ditto for uppercase.

Please note that the 'b' and the 'w' options consume an excessive amount of CPU time, and should be avoided, if possible. HINT: If possible, use the SIZE variable to avoid using patterns with these flags on large messages.

If neither 'h' or 'b' is specified, the pattern is matched against the header only. Specifying the 'b' option causes the pattern to be matched against the message body. Specifying both causes the pattern to be matched against the entire message.

Normally, each line in the message gets matched against the pattern individually. When applying patterns to a header, multi-line headers (headers split on several lines by beginning each continuation line with whitespace) are silently combined into a single line, before the pattern is applied. Specifying the 'w' flag causes the pattern to be applied to the whole part of the message that's being searched (which is specified via the presence, or the absence, of the h and b flags). Also, if the 'w' flag is used, but neither 'h' nor 'b' flags are used, the 'b' flag is the default, instead of the 'h' flag.

This flag also changes the way that ^ and $ is interpreted in patterns. Normally, those characters anchor the pattern at the beginning, or the end, of each line. When the 'w' flag is specified, ^ and $ anchors the pattern against the beginning, or the end, of either the header, body, or both.

Patterns are evaluated by maildrop as any other numerical expression. If a pattern is found, maildrop's filter interprets the results of the pattern match as number 1, or true, for filtering purposes. If a pattern is not found the results of the pattern search is zero. Once a pattern is found, the search stops. Second, and subsequent occurrences of the same pattern are NOT searched for.

maildrop can also do weighted scoring. In weighted scoring, multiple occurrences of the same pattern are used to calculate a numerical score.

To use a weighted search, specify the pattern as follows:

/pattern/:options,xxx,yyy

where xxx and yyy are two numbers. yyy is optional -- it will default to 1, if missing.

The first occurrence of the pattern is evaluated as xxx. The second occurrence of the pattern is evaluated as xxx*yyy, the third as xxx*yyy*yyy, etc... All occurrences of the pattern are added up to calculate the final score. Note:

When the w option is not specified, maildrop does not recognize multiple occurrences of the same pattern in the same line. This is required in order to have the ^ and $ operators working correctly. For example:

/^Received:/:1

This pattern counts how many Received: headers the message has, and does not recognize any occurrences of the text "Received:" anywhere else in the headers.

You must specify the w option in order to take into account multiple occurrences of the same pattern in the message. This also activates all the usual semantics of the 'w' option. For example:

/[:upper:]/:wbD,1

counts the number of uppercase letters in the body of the message.

After a pattern is successfully matched, the actual text that is matched is placed in the MATCH variable. For example:

/^From:.*/

matches a line of the form:

From: postmaster@localhost

Here the variable MATCH will be set to "From: postmaster@localhost", which can be used in subsequent statements. It is possible to use selective parts of the matched string by using the ! character in patterns. For example:

/^From: *!.*/

matched against the same line will set MATCH to "From: " and MATCH2 to "postmaster@localhost". More than one ! character may be used. Subsequent matched text is assigned to MATCH3, MATCH4, and so on. Note: Please note that the abuse of this feature will eat plenty of CPU time -- the ! character introduces a lot of complexity for maildrop to deal with.

When there is more than one way to match a string, maildrop favors matching as much as possible initially. For example:

/^To:.*,!.*/

when matched against

To: joe@somewhere,bob@somewhere.else,gary@whoknowswhere will set MATCH to "To: joe@somewhere,bob@somewhere.else," and MATCH2 to "gary@whoknowswhere".

The MATCH variables are NOT set when weighted scoring is used, since the same pattern is matched multiple times.

Although maildrop evaluates expressions numerically, results of expressions are stored as text literals. When necessary, text literals are converted to numbers, then the results of a mathematical operation is converted back into a text literal.

The following operators carry their usual meaning, and are listed in order from lowest precedence, to the highest:

|| && < <= > >= == != lt le gt ge eq ne | & + - * / =~ /pattern/ /pattern/ ! ~ function()

VARIABLE=expression

Assigns the result of the expression to VARIABLE (note no leading $ in front of variable). Note: If VARIABLE is NOT surrounded by quotes, then it may contain only letters, numbers, underscores, dashes, and a selected few other characters. In order to initialize a variable whose name contains non-standard punctuation marks, surround the name of the variable with quotes.

cc expression

The cc statement is very similar to the to statement, except that after delivering the message maildrop continues to process the filter file, unlike the to statement which immediately terminates maildrop after the delivery is complete. Essentially, the message is carbon copied to the given mailbox, and may be delivered again to another mailbox by another cc or to statement.

See the to statement for more details. When cc is used to deliver a message to a process maildrop will set the EXITCODE variable to the process's exit code.

dotlock expression {

...

}

maildrop automatically creates a lock when a message is delivered to a mailbox. Depending upon your system configuration, maildrop will use either dot-locks, or the flock() system call.

The dotlock statement creates an explicit dot-lock file. Use the flock statement to create an explicit flock() lock.

The expression is a filename that should be used as a lock file. maildrop creates the indicated dot-lock, executes the filtering instructions contained within the { ... } block, and removes the lock. The expression must be the name of the dot-lock file itself, NOT the name of the mailbox file you want to lock. Note:

With manual locking, it is possible to deadlock multiple maildrop processes (or any other processes that try to claim the same locks).

No deadlock detection is possible with dot-locks, and since maildrop automatically refreshes all of its dot-locks regularly, they will never go stale. You'll have maildrop processes hanging in limbo, until their watchdog timers go off, aborting the mail delivery.

echo expression

maildrop will print the given text. This is usually used when maildrop runs in embedded mode, but can be used for debugging purposes. Normally, a newline is printed after the text. If text is terminated with a \c, no newline will be printed.

exception {

...

}

The exception statement traps errors that would normally cause maildrop to terminate. If a fatal error is encountered anywhere within the block of statements enclosed by the exception clause, execution will resume immediately following the exception clause.

exit

The exit statement immediately terminates filtering. maildrop's return code is set to the value of the EXITCODE variable. Normally, maildrop terminates immediately after successfully delivering the message to a mailbox. The exit statement causes maildrop to terminate without delivering the message anywhere.

The exit statement is usually used when maildrop runs in embedded mode, when message delivery instructions are not allowed.

flock expression {

...

}

maildrop automatically creates a lock when a message is delivered to a mailbox. Depending upon your system configuration, maildrop will use either dot-locks, or the flock() system call.

The flock statement creates a manual flock() lock. Use the dotlock statement to create a manual dot-lock file.

The expression is the name of the file that should be locked. maildrop creates the lock on the indicated file, executes the filtering instructions contained within the { ... } block, and removes the lock. Note: With manual locking, it is possible to deadlock multiple maildrop processes (or any other processes that try to claim the same locks). The operating system will automatically break flock() deadlocks. When that happens, one of the maildrop processes will terminate immediately. Use the exception statement in order to trap this exception condition, and execute an alternative set of filtering instructions.

foreach /pattern/:options
{
    ...
}

foreach (expression) =~ /pattern/:options { ... }

The foreach statement executes a block of statements for each occurrence of the given pattern in the given message, or expression. On every iteration MATCH variable will be set to the matched string. All the usual options may be applied to the pattern match, EXCEPT the following:

,xxx,yyy

Weighted scoring is meaningless, in this context.

!

The ! operator in the pattern does not work as expected. For each iteration, only the MATCH variable will be set. If the pattern includes the ! operator, the statements will be executed once for each section in the matched string, with the MATCH variable set to the contents of each section.

if (expression)
{
    ...
}
else
{
    ...
}

Conditional execution. If expression evaluates to a logical true (note - parenthesis are required) then the first set of statements is executed. The else keyword, and the subsequent statements, are optional. If present, and the expression evaluates to a logical false, the else part is executed.

maildrop evaluates all expression as text strings. In the context of a logical expression, an empty string, or the number 0 constitutes a logical false value, anything else is a logical true value.

If the if part, or the else part consists of only one statement, the braces may be omitted. Note: The grammar of this if statement is stricter than usual. If you get baffling syntax errors from maildrop, make sure that the braces, and the if statement, appear on separate lines. Specifically: the closing parenthesis, the closing braces, and the else statement, must be at the end of the line (comments are allowed), and there may not be any blank lines in between (not even ones containing comments only).

import variable

When maildrop starts, it normally imports the contents of the environment variables, and assigns them to internal maildrop variables. For example, if there was an environment variable FOO, the internal maildrop variable FOO will have the contents of the environment variable. From then on, FOO will be no different than any other variable, and when maildrop runs an external command, the contents of maildrop's variables will be exported as the environment for the command.

Certain variables, like HOME and PATH, are always reset to fixed defaults, for security reasons. Also, in delivery and embedded modes, the environment is not imported at all, and maildrop starts with only the fixed default variables.

The import statement initializes the specified variable with the contents of the original environment variable when maildrop started. For example:

echo "PATH is $PATH"
PATH="/bin"
echo "PATH is $PATH"
import PATH
echo "PATH is $PATH"
exit

This results in the following output:

PATH is /bin:/usr/bin:/usr/local/bin
PATH is /bin
PATH is /home/root/bin:/sbin:/usr/sbin:/bin:/usr/bin:/usr/local/bin

This shows that when maildrop starts PATH is set to the fixed default of /bin:/usr/bin:/usr/local/bin. However, the original contents of the PATH environment variable we different, and the import statement shows what it was.

include expression

The include statement reads a file, and executes filtering instructions contained in that file. Note that the include statement is processed when the current filter file is being executed. When maildrop reads the initial filter file, any syntax errors in the filtering instructions are immediately reported, and maildrop will terminate with a return code of EX_TEMPFAIL. Any errors in files specified by include statements are NOT reported, because those files will not be read until the include statement is itself executed.

If the specified file does not exist, or if there are any syntax errors in the file, maildrop reports the error, and terminates with a return code of EX_TEMPFAIL.

logfile expression

log expression

Logging in maildrop is normally turned off. The logfile statement specifies the file where maildrop will log how the message has been disposed of. The parameter is then name of the file. If the file exists maildrop appends to the file.

For each delivery (the to and cc statements, and default deliveries) maildrop records the From: and the Subject: fields, together with the current time, in the log file.

The log statement adds additional logging text to the log file. The log statement works exactly like the echo statement, except that the text is written to the logfile, instead of standard output.

to expression

The to statement delivers the message to a mailbox. expression must evaluate to a valid mailbox. A valid mailbox is either a mailbox file, a maildir, or an external program (which includes forwarding to another address).

The to statement is the final delivery statement. maildrop delivers message, then immediately terminates, with its return code set to the EXITCODE variable. If there was an error while delivering the message, maildrop terminates with the EX_TEMPFAIL exit code. A properly-written mail transport agent should re-queue the message, and re-attempt delivery at some later time.

An expression that begins with the "|" character specifies an external program to run to handle the actual delivery. The SHELL variable specifies the shell to execute the given command. The message is provided to the command on standard input. maildrop's exit code will be the process's exit code.

An expression that begins with an exclamation mark, "!" specifies a whitespace-delimited list of E-mail addresses to forward the message to. The program specified by the SENDMAIL variable is run as an external program, with the list of E-mail addresses provided as parameters to the program.

Otherwise, expression names the mailbox where maildrop delivers the message. If expression is a directory, maildrop assumes that the directory is a maildir directory. Otherwise, maildrop will deliver the message to a file, formatted in traditional mailbox format. maildrop will use either dot-locking, or flock()-locking when delivering the message to the file.

while (expression)
{
    ...
}

The expression is repeatedly evaluated. Each time it evaluates to a logical true, the statements inside the braces are executed. When expression evaluates to a logical false, the while loop is over. Take care to avoid infinite loops.

xfilter expression

expression specifies an external program that maildrop runs to filter the current message. The current message will be piped to the filter program as standard input. The output of the filter program replaces the current message being delivered. The external program must terminate with an exit code of 0. If the external program does not terminate with an exit code of 0, or if it does not read the message from the standard input, maildrop terminates with an exit code of EX_TEMPFAIL.

expression1 || expression2

If expression1 evaluates to a logical true, the result of the || is expression1, otherwise it's expression2, which is evaluated.

maildrop uses the following concept of true/false: an empty text literal, or a text literal that consists of the single character "0" is a logical false value. Anything else is a logical true value.

expression1 && expression2

If expression1 evaluates to a logical false, the result of the && is expression1, otherwise it's expression2, which is evaluated.

maildrop uses the following concept of true/false: an empty text literal, or a text literal that consists of the single character "0" is a logical false value. Anything else is a logical true value.

expression1 < expression2

expression1 <= expression2

expression1 > expression2

expression1 >= expression2

expression1 == expression2

expression1 != expression2

These operators compare their left hand side expression against their right hand side. These operators compare the numerical values of each side, as floating point numbers. If the numbers compare as indicated, the result of the comparison is the text string "1", otherwise it is the text string 0. Note: Ccomparisons are not associative: "a < b < c" is an error. If it is absolutely necessary, use "(a < b) < c".

expression1 lt expression2

expression1 le expression2

expression1 gt expression2

expression1 ge expression2

expression1 eq expression2

expression1 ne expression2

These operators compare their left hand side expression against their right hand side. These operators compare each side as text strings (alphabetically, although the text may include anything). If the text strings compare as indicated, the result of the comparison is the text string "1", otherwise it is the text string 0. Note: Comparisons are not associative: "a lt b lt c" is an error. If it is absolutely necessary, use "(a lt b) lt c". (But why would you?).

expression1 | expression2

This is the bitwise or operator. Its result is a 32 bit integer, which is a bitwise-or combination of the left hand side and the right hand side.

expression1 & expression2

This is the bitwise and operator. Its result is a 32 bit integer, which is a bitwise-and combination of the left hand side and the right hand side.

expression1 + expression2

expression1 - expression2

expression1 * expression2

expression1 / expression2

These are numerical, floating point, operators.

expression =~ /pattern:option

The left hand side of the =~ operator can be any expression. The right hand side is always a pattern specification. The result of the operator is the weighted match of the pattern against expression (if the options do not specify weighted scoring, the result is simply 1 if the pattern was found, 0 if not).

See "Patterns" for more information.

expression =~ /pattern:option

The result of this operator is the weighted match of the pattern against the current message (if the options do not specify weighted scoring, the result is simply 1 if the pattern was found, 0 if not).

See "Patterns" for more information.

! expression

~ expression

The result of the ! operator is a logical opposite of its right hand side expression. If the right hand side expression evaluated to a logical true, the result is a logical false. If it evaluated to a logical false, the result is a logical true.

maildrop uses the following concept of true/false: an empty text literal, or a text literal that consists of the single character "0" is a logical false value. Anything else is a logical true value.

The result of the ~ operator is a bitwise complement of its right hand side expression. The right hand side expression is evaluated as a 32 bit integer, and the result of this operator is a bitwise complement of the result.

escape(expression)

The escape function returns its sole argument with every occurrence of a special character prefixed by a backslash. A special character is any of the following characters:

|!$()[]\+*?.&;`'-~<>^{}"

This can used when matching pattern sections, and then taking one section and matching it again. For example:

if ( /^From: *!.*/ )
{
     MATCH2=escape($MATCH2)
     if ( /^Subject:.*$MATCH2/ )
     {
        ...
     }
}

This example checks if the contents of the From: header can also be found in the Subject: header. If the escape function were not used, then any special characters in the From: header that are also used in regular expressions, such as * or +, would introduce unpredictable behavior, most likely a syntax error.

The reason why this list of special characters also includes characters not used in maildrop's regular expressions is to allow maildrop's variables to be used on the command line of a shell command executed by the xfilter command, backtick characters, or to or cc commands.

Although using data from an external data source is dangerous, and it may result in inadvertent exploits, using the escape function should hopefully result in fewer surprises.

These functions provide support for GDBM database files. See maildropgdbm(5) for more information. Note: The system administrator can disable GDBM support in maildrop, so these commands may not be available to you.

if ( /^From: *!.*/ )
{
     ADDR=getaddr($MATCH2)
}

This function is usually applied to a header that contains RFC 2822 addresses. It extracts the actual addresses from the header, without any comments or extraneous punctuation. Each address is followed by a newline character. For example, if string contains:

joe@domain.com (Joe Brown), "Alex Smith" <alex@domain.com>, tom@domain.com

The result of the getaddr function is the following string:

joe@domain.com<NL>alex@domain.com<NL>tom@domain.com<NL>

Note:

Because getaddr() interprets RFC 2822 loosely, it is not necessary to strip off the "To:" or the "Cc:" header from the string, before feeding it to getaddr(). For example, the following snippet of code takes all addresses in the message, and concatenates them into a single string, separated by spaces:

ADDRLIST=""
foreach /^(To|Cc): .*/
{
    foreach (getaddr $MATCH) =~ /.+/
    {
       ADDRLIST="$ADDRLIST $MATCH"
    }
}

Note: In certain rare situations, RFC 2822 allows spaces to be included in E-mail addresses, so this example is just educational.

if ( hasaddr(string) )
{
   ...
}

"string" is of the form user@domain. The hasaddr function returns 1 if this address is included in any To:, Cc:, Resent-To:, or Resent-Cc:, header in the message, otherwise this function returns 0.

This is more than just a simple text search. Each header is parsed according to RFC822. Addresses found in the header are extracted, ignoring all comments and names. The remaining addresses are checked, and if "string" is one of them, hasaddr returns 1, otherwise it returns 0.

The comparison is case-insensitive. This actually violates RFC822 (and several others) a little bit, because the user part of the address may be (but is not required to be) case sensitive.

if (length(string) > 80)
{
   ...
}

The length function returns the number of characters in string.

if (lookup(expr, file, "option"))
{
   ...
}

expr is any expression. filename is a name of a file containing a list of patterns. Note that filename is relative to the current directory, which is the home directory of the user when maildrop runs in delivery mode, or embedded mode. maildrop then reads the file. Blank lines will be ignored, as well as any lines that begin with the # character (comments).

Leading whitespace (but not trailing whitespace, take care) is removed, and the remaining contents of each line are interpreted as a pattern which is matched against expr. As soon as the match is found, lookup returns "1". If no match is found after reading the entire file, lookup returns "0". For example:

if ( /^To: *!.*/ && lookup( $MATCH2, "badto.dat" ))
{
   exit
}

The file badto.dat contains the following two lines:

friend@public
^[^@]*$

If a message has a To: header that contains the text "friend@public", or does not contain at least one @ character, then the message will be silently dropped on the floor ( maildrop will terminate without delivering the message anywhere).

options are the pattern matching options to use. The only supported option is "D" (the rest are meaningless, in this case). Note: Be careful with discarding messages like that. Pattern matching can be tricky, and a slight miscalculation can cause mail to be unintentionally discarded. It is much desirable to first deliver message to a separate folder or mailbox, and once the filter is verified to work correctly, change it so the messages are discarded completely.

foo=substr($foo, 1, 10)

The substr function takes start number of characters from string. If count is specified, at most count characters starting at position start are kept, any excess is trimmed.

foo=time

The time function returns the current time, in seconds, since January 1, 1970. This function is useful when using GDBM files. See maildropex(5) for an example of using the time function.

foo=tolower(string)

This function returns the string with all uppercase characters replaced by lowercase characters.

foo=toupper(string)

This function returns the string with all lowercase characters replaced by uppercase characters.

The filter file is read by maildrop ($HOME/.mailfilter or another file), and it contains filtering statements, one per line. The filtering language used by maildrop has a loosely - defined grammatical structure.

Statements are listed one per line. Multiple statements may be listed on the same line by separating them with semicolons. To continue a long statement on the next line, terminate the line with a backslash character.