A recent spam where a user collected the addresses from a list of
people who have posted to Usenet groups reminded me of this.  This
particular spammer used Pegasus 2.53 as his email program, which
includes a rather useful item.  Specifically (quoting from an
announcement from David Harris, author of Pegasus mail):

------------- cut here ---------------------------------

Pegasus Mail now adds one of three new headers to messages it sends when
more than 50 recipients are present in the message.  These headers can
be used as filtering triggers to delete such messages.  The headers are
based on the number of recipients, and are as follows:

   0-50 recipients -     no added header
   50-499 recipients -   X-Distribution: Moderate
   500-4999 recipients - X-Distribution: Bulk
   5000+ recipients -    X-Distribution: Mass

These headers have an internal CRC check applied to them and cannot be
changed or omitted by patching the binary.  We have deferred announcing
the presence of these headers until now in order to give WinPMail v2.5x
a chance to propagate widely, and presumably into the hands of a large
number of spammers.  While spammers using older versions of WinPMail
will escape the new headers, they will become progressively further and
further out of date and will be unable to take advantage of the newer
capabilities of the system without upgrading and thus exposing
themselves to automated detection.

------------- resume --------------------------------------

This latest spam indeed did have the X-Distribtion line in the header!

Now I'm not suggesting that Listserv begin using these header lines
(I'm not *not* suggesting it either), but it is one (little) thing
that can be used to filter spams.   Of course, right now, Pegasus
is the only email program I know using these, so the value of checking
for these headers may or may not be all that great.   But, if you
want to include it in your personal filter rules it can't hurt.

Perhaps getting ISP's to use Pegasus as their default distribution
could help (hey, TUCOWS rated it as the best pop program out there, and
it's totally free)?

Is there a way a list could have an exit to pre-filter posts based on
this header (or any other) item?  If so, is there an example (rexx and
perl)?