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I'm in the process of implementing a scheme which:
- Accepts mail if from a recognized source
- Bounces all other mail back, with a rejection letter attached
explaining how to become a "recognized" source (basically resend
the message with a special keyword in the subject). Optionally
logs the action for future reference.
This filters out spam because the spammer is either not going to
bother registering, or will not get the rejection letter in the first
place due to forged headers in the spam message (rejection letters
which bounce back to you are discarded by the filter).
You should check out Spamgard(tm), available at:
ftp://ftp.netcom.com/pub/wj/wje/release/sg-latest.tar.gz
Spamgard(tm) is freely available, does just the type of checking you're
talking about, and is already written.
I've been using it for about six months now. I've gone from about twenty
spams a day, to about one a month that slips through. You can filter on
just about anything you can think of (it's basically a procmail app), and
if you can setup procmail you can set this one up.
The downside of this type of filtering is if you're corresponding with
clueless people that don't understand what they have to do to send you
an email. Typically those addresses end in "@aol.com" and are easily
filtered. ;-)
If you want to see an example of what a user sees if they're not on your
list, just send me an email *without* the keyword "urza" in the subject
line. You'll get a bounce back giving instructions on what to do to get
an email through.
--
Email sent to me will be bounced unread unless the word "urza" is in
the subject line; Disregard if you're on my "correspondent" list.
Spamb0ts please use root(_at_)irs(_dot_)ustreas(_dot_)gov
support(_at_)msn(_dot_)com root(_at_)localhost
Fight spam: http://spam.abuse.net B0t Ba1t: http://www.rafferty.org/yum