[Gllug] Rejecting mail at backup MX

[Xander D Harkness]

Nix wrote:
> On Wed, 11 Feb 2004, Bruce Richardson stipulated:
> 
>>                                   I just use recipient callout, where
>>the mailer connects to the destination system and does this:
>>
>>	helo exim.box
>>	mail from: alleged.sender at alleged.source
>>	rcpt to: recipient at domain.org
>>
>>and rejects the mail if the end system rejects it.
> 
> 
> Er, isn't this what every single relaying MTA on the planet does? :)
> 
The mail server (A) during the SMTP session checks with the next server 
(B) whether it wants to accept the current mail prior to accepting the 
mail and closing the SMTP session. A and B are both run by the same 
company.  There may be multiple duplicate A relays all virus and spam 
scanning so the load is easily distributed, one essential check is that 
there is a valid email account.  The final recipient(s) (B) have an easy 
job of just accepting the mail once the hard work is done.

This means that server A does not get stuck with undeliverable mail and 
especially if it is a virus or spam it does not bounce mail to the wrong 
location.  In addition if more mail servers refuse spam or viruses from 
the original recipient there is the hope that it will cause the 
initiator many more problems.  The other advantage of not accepting the 
mail is that it make the email address look invalid.

Kind regards
Xander
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