Each header filter rule has two parts, a list of regular
expressions, one per line, and an action to take. Mailman
matches the message's headers against every regular expression in
the rule and if any match, the message is rejected, held, or
discarded based on the action you specify. Use <em>Defer</em> to
temporarily disable a rule.
You can have more than one filter rule for your list. In that
case, each rule is matched in turn, with processing stopped after
the first match.
Note that headers are collected from all the attachments
(except for the mailman administrivia message) and
matched against the regular expressions. With this feature,
you can effectively sort out messages with dangerous file
types or file name extensions.