Tool for validating sender address as spam-fighting technique?

[Kelly Jones]

To fight spam, I want to validate the address (not necessarily in
real-time) of the a given email sender. Is there a Unix tool that does
this?

The basics are simple: to validate "kmnyqi at wnonline.net", I connect to
the MX record of wnonline.net and go as far as "RCPT TO" as follows:

> host -t mx wnonline.net
wnonline.net mail is handled by 5 wnspf.bayou.com.

> telnet wnspf.bayou.com. 25
Trying 209.209.192.75...
Connected to wnspf.bayou.com..
Escape character is '^]'.
220 Welcome to Bayou mxfilter
HELO domaintester.com
250 mxfilter.bayou.com
MAIL FROM: <test at ignoreme.com>
250 Ok
RCPT TO: <kmnyqi at wnonline.net>
550 <kmnyqi at wnonline.net>: Recipient address rejected: 5.1.1
<kmnyqi at wnonline.net>... User unknown
QUIT
221 Bye
Connection closed by foreign host.

This tells me kmnyqi at wnonline.net is an invalid address and that mail
from that address is probably bogus.

A more sophisticated tool would cache results, handle temporary
failures (eg, inability to connect to the MX server), handle multiple
MX records, perhaps even publish results [carefully, to avoid giving
spammers a source of legit email addresses!], etc. Plus, I'd prefer to
use a tested tool vs hacking something up myself.

I realize this technique is far from perfect:

Spammers spoof legit addresses

Bounces/Mailing lists/etc legitimately use "do not reply" addresses

It could be considered unfriendly to the target MX servers

Some mail servers incorrectly say "user unknown" when they see spam,
figuring it's more of a deterrent than saying "you're a spammer"

Some mail servers inefficiently accept mail for "foo at xxx.com" (where
xxx.com is one of their domains), figure out if foo exists later, and
send a bounce back to the envelope sender, instead of rejecting email
at the SMTP level (a really good tool would create throwaway addresses
to catch these cases too)

... but I still think it might help.

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