Setting up Backup Mail

In Debian and Fedora (and presumably Red Hat), the default sendmail.mc has the access database enabled. You can use this to add domains that you wish to relay; for each domain for which you wish to be a backup MX, go to the /etc/mail directory, and add a line of the form:

domain.example.com           RELAY

to the access file. Then leave your editor, and use make to build the database. While Debian's makefile will suggest you also do /etc/init.d/sendmail reload after running make, I've found that it's unnecessary in most cases.

The access database can also be used to deny relaying; if there's a zombie running a dictionary attack, you can nip it in the bud with something like:

Connect:192.168.42.42          REJECT

or

From:idiotspammer@example.com            550 Address unknown

More information is available here.

EDIT: pointing out the /etc/mail/access file.