Tool for validating sender address as spam-fighting technique?

Chad Leigh -- Shire.Net LLC chad at
Sun Mar 11 18:41:51 UTC 2007

On Mar 11, 2007, at 6:31 AM, Justin Mason wrote:

> for what it's worth, I would suggest *not* adopting this
> as an anti-spam technique.
> Sender-address verification is _bad_ as an anti-spam technique, in my
> opinion.  Basically, there's one obvious response for spammers  
> looking to
> evade it -- use "real" sender addresses. Where's an easy place to find
> real addresses? On the list of target addresses they're spamming!

This is a red-herring.  They already do that.  They have been doing  
that for a long time.  And it has nothing to do with sender  

Sender verification works and works well.

> Hence, the spam recipients now get twice as much mail from each  
> spam run
> -- spam aimed at them, *and* bounce blowback from hundreds of spams  
> aimed
> at others, forged to appear to be from them.  It's the obvious  
> response to
> SAV, which is one reason why we never implemented something like  
> that in
> SpamAssassin.

Sorry, but you conclusion does not follow.  Sender verification has  
been around for a while and this has not happened in my experience.   
Ie, there is no greater use of real FROM addresses than there was  

Most MTAs have in-built routines to do this, with exim having a  
particularly good facility for this.  Technically, with exim's, you  
are actually validating the sending server's adherence to the RFCs  
about accept DSN replies back.


> --j.
> Kelly Jones writes:
>> 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", 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
>> Trying
>> Connected to
>> Escape character is '^]'.
>> 220 Welcome to Bayou mxfilter
>> 250
>> MAIL FROM: <test at>
>> 250 Ok
>> RCPT TO: <kmnyqi at>
>> 550 <kmnyqi at>: Recipient address rejected: 5.1.1
>> <kmnyqi at>... User unknown
>> 221 Bye
>> Connection closed by foreign host.
>> This tells me kmnyqi at 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" (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.
> _______________________________________________
> freebsd-questions at mailing list
> To unsubscribe, send any mail to "freebsd-questions- 
> unsubscribe at"

Chad Leigh -- Shire.Net LLC
Your Web App and Email hosting provider
chad at

More information about the freebsd-questions mailing list