This program is included with the procmail distribution.
It??™s used to help filter header information from messages. Two header fields are
added to the message header by formail: a Precedence: line and an X-Loop: line. These
lines are used by formail to help prevent message loops. After that, a simple shell
Fighting Spam and Viruses 329
15
command is used to generate a message and redirect it to the local MTA process, which is
defined by the $SENDMAIL environment variable.
The last recipe demonstrates filtering messages based on a Subject header line. Any
message with a subject containing the word ???pills??? is placed in the mail folder /dev/null.
System administrators may recognize this as a special file. The /dev/null file maintains a 0
byte file size. Any information copied there is lost forever. Thus, this recipe deletes any
messages with the word ???pills??? in the Subject line. While this technique can be used for
blocking known spam messages from your email server, it is extremely dangerous. If any
valid emails get caught by this recipe, they are deleted without your knowing it.
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