Blocking mail from bad places

Wed Apr 4 16:46:57 UTC 2007

Greetings.

While its a pretty brute force approach, one method I’m trying is to
curtail the source of email. In otherwords, if smtp traffic comes from an
unknown source it gets directed to a sendmail server that intentionally
rejects the email message (550 with a informational message/url). If the
email message comes from a “known” source (friend/family’s ISP) it
gets routed to my main sendmail server which allows most email after
checking for the obvious (non resolvable domains, blacklisted domains etc)
using an access lists.
I’ve cut down on Spam (including this account which I use solely for
NANOG) to about 0. Granted the amount of valid email that can get rejected
is high, but since I log the bounces on the drop server I can look for
obvious rejects from good/expected email servers.
Not by any means a solution to/for a large even medium size provider, but
for a small home based setup it works well. Details at http://www.sumless.net/nsh.html

Cheers,
-Joe Blanchard