MAILDROP(1) Double Precision, Inc. MAILDROP(1)
maildrop - mail delivery filter/agent
maildrop [option...] [-d user] [arg...]
maildrop [option...] [filename] [arg...]
maildrop is a replacement local mail delivery agent that includes a
mail filtering language. The system administrator can either replace
the existing mail delivery agent with maildrop, or users may run
maildrop using the 'forward to program' mechanism of the existing mail
maildrop first reads the E-mail message on standard input. Trailing
carriage return characters are automatically stripped. An E-mail
message consists of header lines, followed by a blank line, followed by
the contents of the message.
maildrop does not accept an mbox-style From_ line before the first
header line. maildrop does not accept leading empty lines before the
first non-blank header line. If the message can possibly start with
empty lines, and a From_ line, use reformail -f0 to remove any initial
empty lines, and replace a From_ line with a proper "Return-Path:"
header; then pipe it to maildrop.
If the file /etc/courier/maildroprc exists, mail delivery or mail
filtering instructions are read from that file. maildrop's
delivery/filtering instructions may direct maildrop to save the message
in specific mailbox, discard it, return it to sender, or forward it to
a different E-mail address.
If /etc/courier/maildroprc does not exist, or its mail delivery
instructions do not completely dispose of this message, maildrop then
reads the mail delivery instructions from $HOME/.mailfilter. If it
doesn't exist, or its mail delivery instructions do not completely
dispose of the message, maildrop then saves the E-mail message in the
maildrop knows how to deliver mail to an standard mailbox files; it
also knows how to deliver to maildirs. A maildir is a directory-based
mail format used by the Courier and Qmail mail servers. Many
other mail servers also know how to read maildirs. When delivering to
mailbox files, maildrop will lock the mailbox for the duration of the
This is the general mail delivery behavior. There are minor differences
in behavior depending on maildrop delivery mode, which is determined
based on how maildrop was started. maildrop uses three different
primary operating modes:
A file containing filtering instructions - filename is specified as
an argument to the maildrop command. maildrop reads this filename
(after /etc/courier/maildroprc) and follows the instructions in it.
Unless the message is explicitly forwarded, bounced, deleted, or
delivered to a specific mailbox, it will be delivered to the user's
maildrop is the mail server's mail delivery agent. maildrop runs
in delivery mode when no filename is specified on the command line.
maildrop changes the current directory to the user's home
directory, then reads /etc/courier/maildroprc, then
maildrop functions as a part of another application. The embedded
mode is used by the Courier mail server to integrate mail
filtering directly into the process of receiving mail from a remote
mail relay, thus rejecting unwanted mail before it is even accepted
for local mail delivery. Embedded mode is used when either the -m,
or the -M, option is specified, and is described below. See below
for a more extensive description of the embedded mode.
It is safe to install maildrop as a root setuid program. The Courier
mail server installs maildrop as a root setuid program by default,
in order to be able to use maildrop in embedded mode. If root runs
maildrop (or it is setuided to root) the -d option may be used to
specify the message's recipient. maildrop immediately resets its
userid to the one specified by the -d option. The user's
$HOME/.mailfilter is read (if it exists), and the message is delivered
to the indicated user.
The system administrator can configure maildrop to restrict the -d
option for everyone except the mail system itself.
If in delivery mode the user's home directory has the sticky bit set,
maildrop immediately terminates with an exit code of EX_TEMPFAIL,
without doing anything. Mail servers interpret the EX_TEMPFAIL exit
code as a request to reschedule the message for another delivery
attempt later. Setting the sticky bit allows $HOME/.mailfilter to be
edited while temporarily holding all incoming mail.
maildrop also terminates with EX_TEMPFAIL if the user's home directory
has world write permissions.
maildrop immediately terminates with EX_TEMPFAIL if the filename is not
owned by the user, or if it has any group or world permissions. This
includes read permissions. The permissions on $HOME/.mailfilter may
only include read and write privileges to the user.
When using the special embedded mode (see below) maildrop immediately
terminates with the exit code set to EX_TEMPFAIL if $HOME/.mailfilters
is not owned by the user, or if it has any group or world permissions.
maildrop is heavily optimized and tries to use as little resources as
possible. maildrop reads small messages into memory, then filters
and/or delivers the message directly from memory. For larger messages,
maildrop accesses the message directly from the file. If the standard
input is not a file, maildrop writes the message to a temporary file,
then accesses the message from the temporary file. The temporary file
is automatically removed when the message is delivered.
Makes the Courier Authentication Library usage mandatory, i.e.
maildrop will throw a temporary error code if the call to the
authlib mechanism fails for some reason, such as authdaemon being
This setting may already be the default, depending on
-A "Header: value"
Adds an additional header to the message. Specifying -A "Foo: Bar"
effectively adds this header to the message being delivered.
The mail transport agent usually adds additional headers when
delivering a message to a local mailbox. The way it's usually done
is by the mail transport agent sending the message using a pipe to
the local delivery agent - such as maildrop - and adding some
additional headers in the process. Because maildrop receives the
message from a pipe, maildrop must either save the message in
memory or write the message into a temporary file.
The -A option enables the file containing the message to be
provided to maildrop directly, as standard input, and the
additional headers specified on the command line. Because the
standard input is a file, maildrop will not need a temporary file.
Multiple -A options may be specified.
Run maildrop in delivery mode for this user ID.
The system administrator may optionally restrict the -d option to
be available to the mail system only, so it may not be available to
you. In all cases, the -d option is allowed if user is the same
user who is running maildrop. Also, for the -d option to work at
all, maildrop must be executed by root, or maildrop must be a
root-owned program with the setuid bit set. Absence of a filename
on maildrop's command line implies the -d option for the user
If -d is not specified, the first argument following all the
options is a name of the file containing filtering instructions.
The remaining arguments, if any, are assigned to the variables $1,
$2, and so on (see "Environment" and "Variable
Sets the FROM variable (message envelope sender) to address. The
system administrator may optionally disable the -f option for
users, so it may not be available to you.
Run maildrop in embedded mode. It's possible to use both the -m,
and the -d options, but it doesn't make much sense to do so. Even
if you really wanted to run your message through someone else's
.mailfilter, that .mailfilter probably has at least one instruction
which is not allowed in the embedded mode.
The filename argument to maildrop should be specified. filename is
a file that includes filtering instructions to be processed in
embedded mode. The -m option is used for debugging filter files
which are later placed in $HOME/.mailfilters, and used with the -M
Run maildrop in a special embedded mode. The -d option is implied
when -M is used, and if absent it defaults to the userid running
All the requirements for the -d option apply. maildrop must either
be executed by root, or the maildrop program must be owned by root
with the setuid bit set. maildrop immediately gives up root
privileges by changing its user ID to the one specified by -d, then
reads $HOME/.mailfilters/filterfile. For security reasons the name
of the file may not begin with a slash or include periods.
maildrop is very paranoid: both $HOME/.mailfilters, and
$HOME/.mailfilters/filterfile must be owned by the user, and may
not have any group or world permissions.
The -M option allows for some friendly cooperation between the user
running the application, and the user who provides a filter for the
embedded mode. The user running the application can use someone
else's canned filter and be assured that the filter is not going to
run amok and start sending mail or create files all over the place.
The user who provides the filter can be assured that the
environment variables are clean, and that there are no surprises.
maildrop supports the concept of "default" filter files. If the
file specified by the -M option cannot be found in
$HOME/.mailfilters, maildrop will try to open
$HOME/.mailfilters/filterfileprefix-default. filterfileprefix is
the initial part of filterfile up until the last '-' character in
If $HOME/.mailfilters/filterfileprefix-default does not exist, and
there are any other dashes left in filterfileprefix, maildrop
removes the last dash and everything following it, then tries
As a last resort maildrop tries to open $HOME/.mailfilters/default.
For example, if the parameter to the -M option is
mailfilter-lists-maildrop, maildrop will try to open the following
files, in order:
Note that maildrop looks for -default files ONLY if -M is used.
This option is reserved for use by the version of maildrop that
comes integrated with the Courier mail server.
Initialize the VERBOSE variable to level. Because maildrop parses
the entire file before running it, this option is used to produce
debugging output in the parsing phase. Otherwise, if filename has
syntax errors, then no debugging output is possible because the
VERBOSE variable is not yet set.
-V is ignored when maildrop runs in delivery mode.
The -w N option places a warning message into the maildir if the
maildir has a quota setting, and after the message was successfully
delivered the maildir was at least N percent full.
Copy the warning message from filename, or from /etc/quotawarnmsg
if this option is not specified, with the addition of the "Date:"
and "Message-Id:" headers. The warning is repeated every 24 hours
(at least), until the maildir drops below N percent full.
This option is available if maildrop is compiled with optional
Dovecot authentication support. socket specifies the location of
Dovecot master authentication socket, for example
If a filename is not specified on the command line, or if the -d option
is used, maildrop will run in delivery mode. In delivery mode, maildrop
changes to the home directory of the user specified by the -d option
(or the user who is running maildrop if the -d option was not given)
and reads $HOME/.mailfilter for filtering instructions.
$HOME/.mailfilter must be owned by the user, and have no group or
global permissions (maildrop terminates if it does).
If $HOME/.mailfilter does not exist, maildrop will simply deliver the
message to the user's mailbox.
If the file /etc/courier/maildroprc exists, maildrop reads filtering
instructions from this file first, before reading $HOME/.mailfilter.
This allows the system administrator to provide global filtering
instructions for all users.
/etc/courier/maildroprc is read only in delivery mode.
The -d option can also specify a name of a virtual account or mailbox.
See the makeuserdb(1) manual page in the Courier Authentication
library's documentation for more information.
The embedded mode is used when maildrop's filtering abilities are
desired, but no actual mail delivery is needed. In embedded mode
maildrop is executed by another application, and is passed the -m or
the -M option. maildrop reads the message, then runs the filtering
rules specified in filename.
filename may contain any filtering instructions EXCEPT the following:
` ... `
Text strings delimited by back-tick characters (run shell command)
are not allowed.
The cc command is not allowed in embedded mode.
The dotlock command is not allowed in embedded mode.
The flock command is not allowed in embedded mode.
In embedded mode, GDBM databases may be opened only for reading.
The log command is not allowed in embedded mode.
The logfile command is not allowed in embedded mode.
The system command is not allowed in embedded mode.
The to command is not allowed in embedded mode.
The xfilter command is not allowed in embedded mode.
Normally when the filename does not explicitly delivers a message,
maildrop will deliver the message to the user's default mailbox. This
is also disabled in embedded mode.
The filename may communicate with the parent application by using the
echo statement and the EXITCODE environment variable.
If maildrop encounters an include statement where the filename
starts with /etc/courier/maildroprcs/, the normal restrictions for the
embedded mode are suspended while executing the filter file in the
/etc/courier/maildroprcs directory. The restrictions are also suspended
for any additional filter files that are included from
/etc/courier/maildroprcs. The restrictions resume once maildrop
finishes executing the file from /etc/courier/maildroprcs.
This allows the system administrator to have a controlled environment
for running external commands (via the backticks, the system or the
The name of the file may not contain any periods (so that a creative
individual can't write include
Before executing the commands in the /etc/courier/maildroprcs file,
maildrop automatically resets the following variables to their initial
values: DEFAULT, HOME, LOCKEXT, LOCKSLEEP, LOCKTIMEOUT, LOCKREFRESH,
LOGNAME, PATH, SENDMAIL, and SHELL. Please note that the previous
values of these variables (if they were changed) will NOT be restored
once maildrop finishes executing the commands from
maildrop has a watchdog timer that attempts to abort runaway filtering.
If filtering is not complete within a predefined time interval (defined
by the system administrator, usually five minutes), maildrop
Sets user's home directory, and related variables. If NIS/YP is
install, that will be used as well.
Global filtering instructions for delivery mode.
System mailbox (actual directory defined by the system
Program to forward mail (exact program defined by the system
Filtering instructions in delivery mode.
Directory containing files used in special embedded mode.
lockmail(1), maildropfilter(7), makedat(1),
maildropgdbm(7), maildropex(7), reformail(1),
makemime(1), reformime(1), egrep(1), grep(1), , courier(8),
4. "Variable substitution"
5. is passed the -m or the -M option.
Courier Mail Server 07/24/2017 MAILDROP(1)
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