220 mx2.avalonworks.net ESMTP Postfix helo hostname-of-internal-mailserver.mati.ca 250 mx2.avalonworks.net mail from:<email@example.com> 250 Ok rcpt to:<firstname.lastname@example.org> 554 <hostname-of-internal-mailserver.mati.ca>: Helo command rejected: Host not foundEr, sorry, what failed? I guess it forgot to tell me way back then. I thought for a minute that it waited until RCPT to be able to accept mail to postmaster in any case, but no, that fails there too. I particularly like how an unsuccessful DNS lookup causes a permanent mail failure, because DNS is always reliable.
Besides, no-one ever sends mail from a host that can't receive mail (say, behind NAT, maybe), and spammers would never put a resolvable address in HELO.
On a hunch:
220 mx2.avalonworks.net ESMTP Postfix helo mail2.igs.net 250 mx2.avalonworks.net mail from:<email@example.com> 250 Ok rcpt to:<firstname.lastname@example.org> 250 OkYes, spammers would never ever just put the destination address in the HELO. Sometimes I think antispam efforts exist to provoke a cold war. (See also "challenge-response").