11.6. Mail transport, delivery and user agents

Debian packages which process electronic mail, whether mail user agents (MUAs) or mail transport agents (MTAs), must ensure that they are compatible with the configuration decisions below. Failure to do this may result in lost mail, broken From: lines, and other serious brain damage!

The mail spool is /var/mail and the interface to send a mail message is /usr/sbin/sendmail (as per the FHS). On older systems, the mail spool may be physically located in /var/spool/mail, but all access to the mail spool should be via the /var/mail symlink. The mail spool is part of the base system and not part of the MTA package.

All Debian MUAs, MTAs, MDAs and other mailbox accessing programs (such as IMAP daemons) must lock the mailbox in an NFS-safe way. This means that fcntl() locking must be combined with dot locking. To avoid deadlocks, a program should use fcntl() first and dot locking after this, or alternatively implement the two locking methods in a non blocking way. 3 Using the functions maillock and mailunlock provided by the liblockfile* packages is the recommended way to accomplish this.

Mailboxes are generally either mode 600 and owned by user or mode 660 and owned by user:mail. 4 The local system administrator may choose a different permission scheme; packages should not make assumptions about the permission and ownership of mailboxes unless required (such as when creating a new mailbox). A MUA may remove a mailbox (unless it has nonstandard permissions) in which case the MTA or another MUA must recreate it if needed.

The mail spool is 2775 root:mail, and MUAs should be setgid mail to do the locking mentioned above (and must obviously avoid accessing other users’ mailboxes using this privilege).

/etc/aliases is the source file for the system mail aliases (e.g., postmaster, usenet, etc.), it is the one which the sysadmin and postinst scripts may edit. After /etc/aliases is edited the program or human editing it must call newaliases. All MTA packages must come with a newaliases program, even if it does nothing, but older MTA packages did not do this so programs should not fail if newaliases cannot be found. Note that because of this, all MTA packages must have Provides, Conflicts and Replaces: mail-transport-agent control fields.

The convention of writing forward to address in the mailbox itself is not supported. Use a .forward file instead.

The rmail program used by UUCP for incoming mail should be /usr/sbin/rmail. Likewise, rsmtp, for receiving batch-SMTP-over-UUCP, should be /usr/sbin/rsmtp if it is supported.

If your package needs to know what hostname to use on (for example) outgoing news and mail messages which are generated locally, you should use the file /etc/mailname. It will contain the portion after the username and @ (at) sign for email addresses of users on the machine (followed by a newline).

Such a package should check for the existence of this file when it is being configured. If it exists, it should be used without comment, although an MTA’s configuration script may wish to prompt the user even if it finds that this file exists. If the file does not exist, the package should prompt the user for the value (preferably using debconf) and store it in /etc/mailname as well as using it in the package’s configuration. The prompt should make it clear that the name will not just be used by that package. For example, in this situation the inn package could say something like:

Please enter the "mail name" of your system.  This is the hostname portion
of the address to be shown on outgoing news and mail messages.  The
default is syshostname, your system's host name.
Mail name ["syshostname"]:

where syshostname is the output of hostname --fqdn.