dot-courier - Local mail delivery instructions
In most cases delivering mail to an account means simply placing the
message in the account´s system mailbox, but that does not have to be
the case. Alternate mail delivery instructions include running a
separate program to process the message, or forwarding the message to
another address. The various .courier files specify some basic mail
delivery instructions. If sophisticated mail filtering is required, the
delivery instructions should include running an external mail filter,
such as maildrop(1).
The file $HOME/.courier specifies how messages are delivered to this
account. If this file does not exist, default instructions set by the
system administrator are used. The system administrator´s default
instructions specify the location of the account´s system mailbox.
In addition to receiving mail addressed user@domain, it is also
possible for user to receive mail addressed to user-foo@domain, for
arbitrary values of foo. To do this, install $HOME/.courier-foo, with
delivery instructions for mail addressed to user-foo@domain.
The system administrator can configure the Courier mail server to
accept mail without regard to whether addresses are in uppercase and
lowercase. In that case the name of a .courier file must contain only
lowercase characters. In any event, all periods in the address must be
replaced with colons. For example, to specify delivery instructions for
user-Foo.Bar@domain, put the delivery instructions in
The file $HOME/.courier-foo-default specifies delivery instructions for
any user-foo-bar@domain address, where bar can be anything. However, it
does NOT control mail delivery to user-foo@domain, which is controlled
Possible mail delivery instructions include: whether each message
should be delivered to a non-standard mailbox; forwarded to another
E-mail address; or if another program should be executed to handle the
message. Programs executed from a .courier file have access to some
environment variables (see ENVIRONMENT VARIABLES). Programs executed
from a -default file can read those environment variables to determine
the exact E-mail address the message was delivered to.
Default delivery instructions
The /etc/courier/aliasdir directory is searched as the last resort,
when all attempts to figure out how to deliver mail to a local address
/etc/courier/aliasdir´s functionality is very similar to how the alias
account is implemented in Qmail, except that no actual system account
is needed. If <email@example.com> is a local address, and there is no
such system account, nor is there an alias defined for this address,
the Courier mail server attempts to read delivery instructions from
All the usual aspects of .courier deliveries apply. If there is no
account that corresponds to the address <firstname.lastname@example.org>, the
Courier mail server looks for /etc/courier/aliasdir/.courier-user-foo,
then /etc/courier/aliasdir/.courier-user-default, and finally
It therefore follows that you can use
/etc/courier/aliasdir/.courier-default to specify local mail delivery
instructions for addresses that do not exist. Combined with dynamic
mail delivery instructions (see below), that´s one way to specify
non-standard locations of mailboxes.
The directory /etc/courier/aliasdir/.courier-:xalias/ is created and
maintained by the makealiases(8) script to implement aliases that
deliver directly to programs or mailboxes. See makealiases(8) for
more information. (This directory corresponds to local addresses that
begin with ".xalias/", but the Courier mail server prohibits explicit
local addresses that begin with a period).
Additionally, makealiases(8) creates subdirectories named
/etc/courier/aliasdir/.courier-:xalias-protocol/, where "protocol" is
set by the -m option.
Each .courier file specifies zero or more delivery instructions. If the
.courier file is zero bytes long, it means that default mail delivery
instructions set by the system administrator should be used. If the
file is not a zero length file, and does not specify any delivery
instructions, messages to the corresponding E-mail address are silently
If $HOME/.courier does not exist, it is treated as a zero-length
file, resulting in a delivery to a default mailbox. If
$HOME/.courier-foo does not exist, it is treated as a non-existent
address, returning the message as undeliverable.
If home directories have global read and execute permissions, the
Courier mail server will be able to reject mail to non-existent
mailboxes right away. the Courier mail server´s ESMTP server runs as a
non-privileged process. It will not be able to access home directories
which do not have global read and execute permissions. Therefore, the
message will be accepted for delivery, by the Courier mail server. As
soon as an attempt to deliver the message is made, the missing .courier
file will result in the message being returned as undeliverable.
However, here the Courier mail server has to accept the message for
delivery first, before generating a non-delivery report.
Delivery instructions in .courier are executed one at a time. If the
execution of a delivery instruction fails for some reason, the message
is either returned as undeliverable, or requeued for another delivery
attempt. Messages that remain queued for a long period of time are
returned as undeliverable.
Even if one delivery instruction fails (and the message is returned
as undeliverable) previous delivery instructions in the file will
have been completed anyway.
Blank lines in the file are ignored. Lines starting with the #
character are comments, and are also ignored. Otherwise, each line
specifies one of three possible delivery instructions: deliver to a
system mailbox or a Maildir; run an external program; or forward the
message to another address.
DELIVERY TO A SYSTEM MAILBOX OR A MAILDIR
Lines that start with the . or the / character specify a mailbox or a
Maildir delivery. The line must specify the complete location of the
mailbox file, or a Maildir. Filenames starting with . are relative to
the account´s home directory. A mailbox file is a traditional mailbox
file that´s readable by most mail software. A Maildir is a directory
based mail storage format that offers several advantages over mailbox
files. Mailbox files must be locked, and therefore they do not permit
concurrent mail deliveries. The mailbox file must be locked while a new
message is appended to it, otherwise multiple messages being delivered
at the same time will trample all over each other. Maildirs do not
require locking, and multiple concurrent deliveries can be made to the
same Maildir. You can create Maildirs by using the maildirmake(1)
The Courier mail server does not implement the "dot-locking" form
of mailbox file locking. The Courier mail server´s locking
abilities are limited solely to system file locking facilities
(namely the lockf, or flock system calls). You can always use
maildrop(1), which offers additional locking options.
RUNNING AN EXTERNAL PROGRAM
Lines that begin with a single | character run an external program. The
rest of the line specifies the command to be executed by the shell.
Long commands can be continued on another line by terminating the
previous line with the \ character.
The Courier mail server runs the specified command, and provides the
contents of the message on standard input.
The Courier mail server waits until the external command completes
execution before going to the next delivery instruction. The Courier
mail server examines the exit code of the external command in order to
determine whether the delivery failed, or not.
If the external command terminates with the exit code of zero, the next
delivery instruction is executed. If the command was the last delivery
instruction in the file, the message is considered to be successfully
If the external command terminates with the exit code of 99, any
additional delivery instructions in the file are NOT executed, but the
message is considered to be successfully delivered.
If the external command terminates with any of the following exit
codes: 64, 65, 67, 68, 69, 70, 76, 77, 78, 100, or 112, the E-mail
message will be returned as undeliverable, and no further delivery
instructions will take place.
If the external command terminates with any other exit code, it is
interpreted as a temporary error, and the message will be requeued for
another delivery attempt later.
On subsequent delivery attempts, delivery instructions will be
carried out from the beginning of the .courier file.
DYNAMIC DELIVERY INSTRUCTIONS
Lines that begin with the || characters also run an external program.
The rest of the line specifies the command to be executed by the shell.
Long commands can be continued on another line by terminating the
previous line with the \ character.
However, programs that are executed by the || instruction, unlike |,
have their standard output captured, and reinterpreted as additional
delivery instructions to be carried out. This feature allows an
external program to be invoked to generate dynamic delivery
instructions to be carried out by the Courier mail server.
The standard output of the external program is read and parsed as if it
contained .courier delivery instructions. There´s a fixed upper limit
on the number of bytes in dynamically-generated delivery instructions.
For glibc, the limit is 8191 bytes, other systems´s upper limit should
The dynamically generated delivery instructions may also specify ||
instructions, recursively. There is an upper limit of four recursive
dynamically-generated delivery instructions.
The exit code of the program invoked by the || instructions are
interpreted exactly like the exit code of a program invoked by |, with
the following exceptions. Dynamically-generated delivery instructions
are carried out only if the external program terminates with an exit
code of 0 or 99. Any other exit code discards any dynamically-generated
delivery instructions. All other aspects of exit code treatment of
external programs remains the same. If the exit code is 99, the
delivery is deemed to be successful, and any additional instructions in
the original .courier file are ignored. If the exit code is 0, the
remaining instructions in the original .courier file are executed.
When the Courier mail server delivers to default delivery instructions
in /etc/courier/aliasdir, those delivery instructions are carried out
under the Courier mail server´s installed system user and group id.
That means that any executed programs or mailboxes are accessed as the
Courier mail server´s mail system user and group.
External commands executed from the .courier file will have the
following environment variables:
The home directory.
The recipient´s userid.
The message envelope return address.
The complete receipient address.
When RECIPIENT is of the form user@domain, HOST contains the domain
part of the address.
When RECIPIENT is of the form user@domain, LOCAL contains the user
part of the address.
When USER is of the form $USER-foobar, EXT will contain the foobar
The portion of EXT that follows the first dash.
The portion of EXT2 that follows the first dash.
The portion of EXT3 that follows the first dash.
When delivery instructions for the address user-foo-bar@domain come
from the file $HOME/.courier-foo-default, DEFAULT will contain the
This environment variable contains the entire From_ header that
should be prepended to the message if it is to be delivered to a
This environment variable contains the entire Return-Path: header.
This environment variable contains the entire Delivered-To: header.
When the external program reads the message from standard input,
the message will NOT have the customary From_, Return-Path:, and
Delivered-To: headers which are customary for locally-delivered
messages. The external program can find those headers in the
respective environment variables. If you have a command that
expects to see those headers as a part of the message, you can use
the preline(1) wrapper to add them to the message. For example,
the procmail mail filter requires those headers.
The maildrop mail filter will not require preline if the system
administrator correctly configures the Courier mail server. The
system administrator can optionally configure the Courier mail
server to recognize maildrop, and activate certain
maildrop-specific optimizations in the Courier mail server. If
these arrangemenets have been made, you can run maildrop directly
from the .courier file, in a straightforward fashion, but those
headers will automatically appear in the message, as seen by
maildrop. Because the message is provided directly on standard
input, without using a pipe, maildrop will be able to deliver the
message directly from the Courier mail server´s message queue,
without using a temporary file.
Lines that do not start with the ., /, or the | character specify a
comma-separated list of E-mail addresses to forward the message to. If
the line starts with either the & or the ! character, the character is
ignored; this is a legacy compatibility option.
The Courier mail server´s .courier may seem to be exactly like Qmail´s
.qmail, but there are some minor differences. Qmail, as of 1.03, does
not implement dynamic delivery instructions. The Courier mail server
also uses a slightly different set of return codes which are classified
as hard errors. The Courier mail server´s implementation of forwarding
differs from Qmail´s. According to Qmail´s documentation, if any
external command terminates in a permanent or temporary failure, the
message is not forwarded to any forwarding address in the .qmail file,
even to addresses that precede the failed delivery instruction. The
message is forwarded only after it is successfully delivered. The
Courier mail server forwards messages to addresses immediately. Also,
in some cases Qmail resets the return address on the message to the
address of the account being forwarded.
To make things more confusing, there is a configuration setting to have
the Courier mail server read $HOME/.qmail files, instead of
dot-forward(1), maildirmake(1), maildrop(1), courier(8).