Creating an m4 DOMAIN File (Part 2):

The second define sets the maximum number of bytes allowed for all headers on any one piece of mail. By default, no limit is set. In the listing, the maximum length is set to 32,768 bytes (32KB), which is more than enough for any reasonable set of headers. Headers longer than that might indicate a mail problem or some form of mail abuse. So we will keep this setting.

The FEATURE(redirect) macro adds support for the .REDIRECT pseudo-domain. The .REDIRECT pseudo-domain handles mail for people who no longer read mail at your site, but who still get mail sent to an old address. After enabling this feature, add aliases for each obsolete mailing address in the form:

old-address    new-address.REDIRECT

For example, assume that Jay Henson is no longer a valid e-mail user in your domain. His old username, jay, should no longer accept mail. His new mailing address is HensonJ@industry.com. Enter the following alias in the /etc/aliases file:

jay       HensonJ@industry.com.REDIRECT

Now when mail is addressed to the jay account, the following error is returned to the sender telling them to try a new address for the recipient:

551 User not local; please try <HensonJ@industry.com>

This seems like a useful feature, so we keep it in the configuration.

The next line in the file also defines a useful feature. FEATURE(use_cw_file) is equivalent to the Fw/etc/local-host-names command in the sendmail.cf file. As described earlier, the local-host-names file provides a means for defining host aliases, which allow a mail server to accept mail addressed to other hosts.