attomail - minimal MDA with Maildir support

attomail is a minimal Mail Delivery Agent (MDA) for local mail. Mail is accepted from standard input and placed in a Maildir box of a user. This software is intended for use on a single-user machine.

Remote delivery, daemonizing, sender verification, etc. is not implemented and won't be implemented due to its complexity. attomail is not written because mail software did not exist, but because existing software were too large for the simple task of delivering cron mail to the local user.

The workflow of attomail:

1. Create a new file with a unique filename in the mail directory.
2. Write a Received header to the file. If From and Date headers are missing, then they will also be appended to the headers.
3. Pass data from standard input to the file.
4. Exit.

attomail can replace the sendmail binary, but note that delivery is only possible for a single user. When invoked as newaliases or mailq, the program exits with a zero status code. Most options of sendmail are ignored except for the -fname and address arguments. Only the -bm mode (read from stdin and deliver the usual way) is supported, attomail will exit in other modes.

Installation

The user to deliver mail to has to be specified at compile time:

make USERNAME=peter attomail

By default, the Maildir directory is ~/Maildir. It can be changed to ~/.Maildir/inbox as follows:

make USERNAME=peter MAILBOX_PATH=.Maildir/inbox attomail

Absolute paths are also supported. The following configuration will put mail in /var/mail/new/(filename):

make USERNAME=nobody MAILBOX_PATH=/var/mail attomail

To install attomail on your system with the appropriate capabilities:

# Copy attomail binary to its destination
# Make sure attomail is owned by the correct user (i.e. the same as used for USERNAME variable while building it)
chmod 4111 attomail

Note: The attomail binary must be installed with setuid enabled. Else, it won't work.

Usage

If you do not have appropriate privileges to install attomail (you are not root) or if you want to try it out before installing, then you can specify the program as sendmail program for the mail program (from heirloom-mailx).

Example (assuming that attomail is built and available in the current working directory):

echo Testing... | mail -S sendmail=attomail -s Subject peter

Bugs

Not much validation is done for the address specified with the -f option or the recipient address. The mail body is passed unprocessed. If attomail is invoked without specifying mail contents, an empty message will be created. If the mail does not contain headers, Date and From headers will be appended anyway.

Other bugs can be reported in the "Issues" of this Github project.

Copyright

Copyright (c) 2013-2015 Peter Wu <[email protected]>

License: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>. This is free software: you are free to change and redistribute it. There is NO WARRANTY, to the extent permitted by law.