Yes postfix has an option for that. My mailserver does it, I think, if
the client makes any mistakes it will temporarily delay the message. I'd
have to check the configuration in detail to remember what it does.
Most spammer botnets make mistakes with the SMTP protocol so it may be
somewhat useful.
-
Martin

Tamas Rudnai wrote:
> There was a technique - unfortunately I cannot recall how it was called.
> Basically the MTA sent back a busy flag for each incoming mail, and in the
> meantime it stored the sender and other info. If the sender wanted to resend
> the mail after couple of minutes, then it received and treated as normal
> mail, if not then it was a spam - as spammers does not bother to resend
> mails...simply because they had just too much to deal with. Maybe picture is
> now changed though as many malware used for sending spam nowadays instead of
> dedicated spam servers, so that innocent clients sending mails and they
> might have better implementation of the transfer protocol so not sure if
> that still works (each client have to deal with much less outgoing mails).
> 
> Tamas
> 
> 
> 
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