Rather than adding content filtering to a mail system, you could always opt for an email system that has the filtering built-in, such as Ipswitch's iMail Server.
A much cheaper alternative to Microsoft Exchange, iMail is a good choice of mail server for smaller companies. Having additional tools to monitor email makes it seem all the more attractive.
Unfortunately, when you get into the options it is all rather basic, and centers around anti-spam. While having the facility is useful, the anti-spam merely marks detected emails with X-Headers. You have to set inbound rules to detect these headers and perform the appropriate action. And there's no simple way to do it, so it's a long process to manually enter each inbound rule.
The anti-spam filter works through DNS block lists, so it won't detect mail sent from a user who is not on the list. This means that iMail is not hard to circumvent.
iMail also has rather basic inbound and outbound rules, which can detect, for example, outgoing messages with bad language in them. Ipswitch provides downloadable word lists, so you don't have to manually create them.
However, we found it easy enough to build our own rule, adding words relating to a project and have the mail server block those messages.
The rules are not as flexible as in dedicated packages, however. For example, you can only choose to perform one action on a detected mail per rule, such as deleting the email. Others tested here did more.
The rules also do not cover the breadth of other applications on test. It is difficult, for example, to enforce a corporate policy, such as adding a disclaimer and blocking dangerous-looking attachments, because Collaboration Suite does not support these features.
Before we are too harsh on iMail, we have to remember that it is a mail server first. Its content filtering is basic, but it is far ahead of its direct competition. If you are on a tight budget, you will have some protection. But if you can stretch to a dedicated package, you'll be much happier with the results.