A content filter is a utility that scans the headers and body of an email message, and usually takes some action based on what it finds. The most common examples are anti-virus and anti-spam programs. Viruses are commonly spread within the contents of email messages, and if you cannot detect spam based on the connecting client or envelope information, you might have better luck by inspecting the actual contents of a message. Filters might change messages, redirect them, respond to them, or tag them for later processing by another tool.
In this chapter we'll look at content filtering at your mail server, although that may not always be your best option for filtering. MTA filtering is appropriate for filtering that should occur with all or nearly all messages. If you need filtering that is configurable by user, the MTA is not the best choice for it. Other types of filtering to consider are:
Configurable MDAs such as procmail or sieve allow users to manage their own delivery configuration files. Generally, MDAs expect your users to edit their own configuration files on the mail server system. If they don't have system accounts, you must provide another means for them to configure their filtering, such as through a web-based application.
You might also consider allowing your users to take advantage of filtering capabilities within their email clients. If their client packages support filtering, this is an excellent ...