NAME
dot-courier - Local mail delivery instructions
SYNOPSIS
$HOME/.courier
$HOME/.courier-foo
/etc/courier/aliasdir/.courier-foo
DESCRIPTION
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) [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 ~user/.courier-foo:bar.
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 by $HOME/.courier-foo.
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 have failed.
/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 <user [AT] 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 /etc/courier/aliasdir/.courier-user.
All the usual aspects of .courier deliveries apply. If there is no account that corresponds to the address <user-foo [AT] example.com>, the Courier mail server looks for /etc/courier/aliasdir/.courier-user-foo, then /etc/courier/aliasdir/.courier-user-default, and finally /etc/courier/aliasdir/.courier-default.
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.
Program/mailbox
aliases
The directory /etc/courier/aliasdir/.courier-:xalias/ is
created and maintained by the
makealiases(8)
[2]
script to implement aliases that
deliver directly to programs or mailboxes. See
makealiases(8)
[2]
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) [2] creates subdirectories named /etc/courier/aliasdir/.courier-:xalias-protocol/, where "protocol" is set by the -m option.
DELIVERY
INSTRUCTIONS
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 discarded.
Note
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.
Note
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)
[3]
command.
Note
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)
[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 delivered.
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 the “EX_SOFTWARE” exit code, which is usually 70, on most platforms, the E-mail message gets returned as undeliverable, a non-delivery report, and no further delivery instructions takes place.
If the external command terminates with any of the following exit codes: 64, 65, 67, 68, 69, 76, 77, 78, 100, or 112, the delivery attempt is considered to be failed, and the next course of action gets selected by Courier mail server's backscatter suppression settings, as described in the “Backscatter Suppression [4] ” section of the installation instructions; see courier(8) [5] for more information.
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.
Note
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 be similar.
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.
Alias-based
deliveries
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.
ENVIRONMENT
VARIABLES
External commands executed from the .courier file will have
the following environment variables:
HOME
The home directory.
USER
The recipient's userid.
SENDER
The message envelope return address.
RECIPIENT
The complete receipient address.
HOST
When RECIPIENT is of the form user@domain, HOST contains the domain part of the address.
LOCAL
When RECIPIENT is of the form user@domain, LOCAL contains the user part of the address.
EXT
When USER is of the form $USER-foobar, EXT will contain the foobar part.
EXT2
The portion of EXT that follows the first dash.
EXT3
The portion of EXT2 that follows the first dash.
EXT4
The portion of EXT3 that follows the first dash.
DEFAULT
When delivery instructions for the address user-foo-bar@domain come from the file $HOME/.courier-foo-default, DEFAULT will contain the bar part.
UFLINE
This environment variable contains the entire From_ header that should be prepended to the message if it is to be delivered to a mailbox.
RPLINE
This environment variable contains the entire Return-Path: header.
DTLINE
This environment variable contains the entire Delivered-To: header.
Note
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)
[6]
wrapper to add them to the message.
For example, the procmail mail filter requires those
headers.
Note
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.
FORWARDING
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.
BUGS
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 $HOME/.courier.
SEE ALSO
dot-forward(1) [7] , maildirmake(1) [3] , maildrop(1) [1] , courier(8) [5] .
AUTHOR
Sam Varshavchik
Author
NOTES
1. |
http://www.courier-mta.org/maildrop.html
2. |
http://www.courier-mta.org/makealiases.html
3. |
http://www.courier-mta.org/maildirmake.html
4. |
Backscatter Suppression |
http://www.courier-mta.org/install.html#backscatter
5. |
http://www.courier-mta.org/courier.html
6. |
http://www.courier-mta.org/preline.html
7. |