Message Transfer Agents¶

Most modern MTAs including sendmail, postfix and exim support a .forward file for directing incoming mail. You should include the line

in your .forward file in your home directory. This will cause your MTA to invoke slocal on your behalf when a message arrives.

The Maildelivery File¶

The `.maildelivery' file controls how slocal filters and delivers incoming mail. Each line of this file consists of five fields, separated by white-space or comma. Since double-quotes are honored, these characters may be included in a single argument by enclosing the entire argument in double-quotes. A double-quote can be included by preceding it with a backslash. Lines beginning with `#' and blank lines are ignored.

The format of each line in the `.maildelivery' file is:

header:

pattern:

action:

result:

Security of Delivery Files¶

In order to prevent security problems, the `.maildelivery' file must be owned either by the user or by root, and must be writable only by the owner. If this is not the case, the file is not read.

If the `.maildelivery' file cannot be found, or does not perform an action which delivers the message, then slocal will check for a global delivery file at /etc/mmh/maildelivery. This file is read according to the same rules. This file must be owned by the root and must be writable only by the root.

If a global delivery file cannot be found or does not perform an action which delivers the message, then standard delivery to the user's maildrop is performed.

Example Delivery File¶

To summarize, here's an example delivery file:

#
# .maildelivery file for mmh's slocal
#
# Blank lines and lines beginning with a '#' are ignored
#
# FIELD   PATTERN   ACTION  RESULT  STRING
#
# File mail with foobar in the `To:' line into file foobar.log
To        foobar    file    A       foobar.log
# Pipe messages from coleman to the program message-archive
From      coleman   pipe    A       /bin/message-archive
# Anything to the `nmh-workers' mailing list is put in
# its own folder, if not filed already
To        nmh-workers  folder ?     nmh-workers
# Anything with Unix in the subject is put into
# the file unix-mail
Subject   unix      file    A       unix-mail
# I don't want to read mail from Steve, so destroy it
From      steve     destroy A       -
# Put anything not matched yet into mailbox
default   -        file    ?       mailbox

Sub-process environment¶

When a process is invoked, its environment is: the user/group-ids are set to recipient's ids; the working directory is the recipient's home directory; the umask is 0077; the process has no /dev/tty; the standard input is set to the message; the standard output and diagnostic output are set to /dev/null; all other file-descriptors are closed; the environment variables $USER, $HOME, $SHELL are set appropriately, $PATH is preserved, but no other environment variables exist.

The process is given a certain amount of time to execute. If the process does not exit within this limit, the process will be terminated with extreme prejudice. The amount of time is calculated as ((size / 60) + 300) seconds, where size is the number of bytes in the message (with 30 minutes the maximum time allowed).

The exit status of the process is consulted in determining the success of the action. An exit status of zero means that the action succeeded. Any other exit status (or abnormal termination) means that the action failed.

In order to avoid any time limitations, you might implement a process that began by fork()-ing. The parent would return the appropriate value immediately, and the child could continue on, doing whatever it wanted for as long as it wanted. This approach is somewhat risky if the parent is going to return an exit status of zero. If the parent is going to return a non-zero exit status, then this approach can lead to quicker delivery into your maildrop.