From a local ISP; I’m sure this seemed like a great idea at the time.
220 mx2.avalonworks.net ESMTP Postfix helo hostname-of-internal-mailserver.mati.ca 250 mx2.avalonworks.net mail from:<firstname.lastname@example.org> 250 Ok rcpt to:<email@example.com> 554 <hostname-of-internal-mailserver.mati.ca>: Helo command rejected: Host not found
Er, 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:<firstname.lastname@example.org> 250 Ok rcpt to:<email@example.com> 250 Ok
Yes, 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”).