sendmail, 4th Edition

Check Validity of Received:

The Received: header traces the succession of hosts that an email message passes through. One technique used by spam messages is to create false Received: headers both to mask the real identity of the original sending host, and to divert blame to some innocent site. One form of bad Received: header that appears in spam messages looks like this:

Received: from ...............................................................
........................................................................
........................................................................
........................................................................
........................................................................!

This form of Received: header was popular with spam software for a few months, then fell out of favor. The following rule shows one way of dealing with such headers:

LOCAL_RULESETS
H*: $>ScreenForDots

SScreenForDots
R $+ .......... $*     $#error $: "553 Ten or more dots begin " $&{hdr_name} "header"

Here, the LOCAL_RULESETS part of your mc file begins with an unusual-looking H configuration command. The H* is special (H* a Default for All Headers on page 1134) because it matches all headers. When sendmail screens headers, it first calls each rule set specified for a specific named header (as with Subject: in the previous section). If no rule set exists for a particular header name, sendmail next looks for the special definition H* and, if found, passes ...