While the term is very generic, "ContentScanner" usually refers to tools that integrate with an MTA and scan Email on-the-fly as it arrives or goes out. They can be configured to react to Email in any number of ways depending on headers, body text, attachment types or filenames, and so on. The most common use is to hook anti-Virus and anti-Spam tools into the mail sending and delivery process, but they can also drive (selective or not) automated archiving or backup tools, gather statistics, or do just about anything you can think of. Depending on configuration and how they integrate with the MTA of choice they may be able to scan only locally-sent or -received Email or also messages being relayed.

For examples, see MailScanner, QmailScanner and Lowth'sProtector.