This concludes this brief look at mail servers. Although we've tried to pack as much information into a single chapter as possible, this is not meant to be a replacement for books dedicated to setting up and configuring sendmail. Security requirements differ from server to server. Some users may need nothing more than one or two autoresponders; others may run email servers with hundreds of accounts. Who has access to your machine and what they're using it for makes a big difference in how you approach MTA security.
No matter how you choose to configure your server, keep in mind that many security holes come from permission problems. Sendmail, no matter how carefully configured, becomes a security nightmare without the proper file and directory permissions. (As a rule of thumb, make sure that no MTA configuration files are group writable.) If you've been meticulous while setting up your server, you're already well on your way to a secure mail server.