The Book of Postfix by Ralf Hildebrandt and Patrick Koetter
Anyone who has tried to run an e-mail server knows that mail isn't a polite relay race any more. It's a game of smash-mouth football. Nine out of ten times someone opens an SMTP connection to you, it's not with something you want. And as if coping with spammers, viruses and other people's misconfigured mail software wasn't enough, now e-mail is a mission-critical company IT service and is expected to plug in to the LDAP directory. We can't blame you if you decide to outsource mail entirely.
If you do decide to stay on as postmaster@ and fight it out, whatever you do, don't try it with one of last decade's mail books. Although any of the current mail servers, correctly configured, can put up a good fight against the spammers and other bad people, The Book of Postfix by itself is a good reason to make Postfix your mail server of choice. Look here for a good explanation of the SMTP protocol, essential for any mail admin, along with enough detail on the architecture of Postfix to help you really understand the config files. It also offers real-world advice for putting together a mail server setup that is reliable in the face of the spam and virus blitz.
Postfix offers you a lot of choice in where to add filters, sanity checks and other protective countermeasures to your mail server. For example, do you want to set up a content_filter or an smtpd_proxy_filter? Besides offering a cookbook for each solution, The Book of Postfix helps you consider the pros and cons of each feature you're considering. A helpful plus is diagrams illustrating where exactly countermeasures fit into the Postfix architecture.
Postfix is complicated enough on its own, as it divides functionality among multiple processes for security. In order to add spam-fighting tools and have everything work, you need a good understanding of what plugs in to what and how, and this book is a great way to get it.
Downloadable scripts and errata, some of which could save you a late night of troubleshooting, are available at www.postfix-book.com.