Postfix: The Definitive Guide

Content-Checking

The last chance you have to reject a message from Postfix directly is by checking the contents of the message itself. Postfix offers simple content checking through the parameters:

• header_checks for message headers

• mime_header_checks for MIME headers

• nested_header_checks for attached message headers

• body_checks for the body of a message

These checks are an all-or-nothing feature with Postfix. There is no way to bypass checks for certain senders or recipients. For more sophisticated analysis, you should use a separate content filter specifically designed to detect spam. See Chapter 14 for more information on using filters with Postfix.

Each parameter points to a lookup table containing regular expression patterns and actions. The patterns are compared to strings within email messages. If Postfix finds a match, the specified action is executed. By default regular expression checking is not case-sensitive. See Chapter 4 for information on using regular expressions with Postfix lookup tables.

Content Checking Configuration

By default mime_header_checks and nested_header_checks use the same lookup tables as header_checks. If you want to distinguish checks for each one, you can configure them separately; otherwise, configuring header_checks causes mime_header_checks and nested_header_checks to use the same patterns as header_checks. When you assign the checking parameters, indicate both the lookup table and which type of regular expression you are using (see Chapter 4 ...