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

The basics are simple: to validate "[EMAIL PROTECTED]", I connect to
the MX record of and go as far as "RCPT TO" as follows:

host -t mx mail is handled by 5

telnet 25
Connected to
Escape character is '^]'.
220 Welcome to Bayou mxfilter
250 Ok
550 <[EMAIL PROTECTED]>: Recipient address rejected: 5.1.1
<[EMAIL PROTECTED]>... User unknown
221 Bye
Connection closed by foreign host.

This tells me [EMAIL PROTECTED] 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 "[EMAIL PROTECTED]" (where 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.

We're just a Bunch Of Regular Guys, a collective group that's trying
to understand and assimilate technology. We feel that resistance to
new ideas and technology is unwise and ultimately futile.
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