OT: stopping spam with "421 Message rejected"?

[Jan Agermose]

Hi

I've noticed that a lot of mails on some of our mailservers are queued
and resend/retried A LOT because of 

1. its prob. spam
2. the reciver is "rejecting" the mails with a 421 message

and that's just a temp. error, correct? Looks like it as the mails are
set in the retry queue on our mailserver. Im sure this is some sysadmin'
clever "strike back" idea, but to me it seams like its hitting the
middle man :-| 

its not that we are a openrelay, simply that some customers are hosting
domains on our servers, where they have setup forwarding rules to other
mailboxes. And yes, have not enabled a spamfilter on our end :-)

But am I misunderstanding what is going on here or is it OK to reject
spam mails using a temp. error message - are there any rules on that? 

Best regards
Jan


80.160.76.XXX   [00000B68] Fri, 03 Aug 2007 00:00:28 +0200 Client
session <<< 421 Message rejected
80.160.76.XXX   [00000B68] Fri, 03 Aug 2007 00:00:28 +0200 Client
session *** <customorthorm at crestcafe.net> <XXXX at YYYYY.ZZ> 1 1986
00:00:00 ERROR
80.160.76.XXX   [00000B68] Fri, 03 Aug 2007 00:00:28 +0200 Client
session >>> QUIT
SYSTEM          [00000B68] Fri, 03 Aug 2007 00:00:28 +0200 Client
session Disconnected