Mailing List Message #15395
From: Bill Cole <>
Subject: Re: Greylisted ?
Date: Tue, 9 Jan 2007 10:22:06 -0500
To: SIMS Discussions <>
At 9:09 AM +0100 1/9/07, Jorge Chamorro  imposed structure on a stream of electrons, yielding:

Lately I started to receive this kind of messages from certain smtp

Failed to deliver your message to
SMTP: Address rejected by host
Host '' says:
450 <>: Recipient address rejected: Greylisted, see

It seems (after reading the whitepaper in the above link) to be a matter of
re-sending it in a few minutes (?).
Therefore, is it enough to re-configure the smtp settings so as to retry
several times over a longer period of time ?

It should be, but SIMS has some difficulties with some greylisting implementations because it does not understand the specific 4xx response code used as being a temporary failure. If memory serves me, the last dev version  of SIMS  (1.8b9d14) should not have a problem with 450 responses to RCPT.

It is not robust (and never has been) to have an SMTP server retry messages that get 4xx 'temp fail' responses more frequently than every 5 minutes, less frequently than every 30 minutes, or for less than 12 hours total.

Anyone has experienced this, and knows a workaround ?

Greylisting is designed to allow normally behaving mail servers to work normally after an initial delay. The idea is that real mail servers (in contrast with the trojan spamware sending most spam these days) have requeue/retry strategies that will have the same message coming back again in a fairly short time (5-30 minutes) from the same IP address if the sending system is legitimate, and even for the quasi-legit spammers who use their own real mail systems on static IP's, a forced retry adds a chance for external blacklists to notice the spammer. For many greylisting systems, the delay only has to happen for one message as long as you are exchanging mail with the site using greylisting relatively often.  It works very well, except with oddball mail systems that don't retry in the range of frequencies and lifetimes as the bulk of mail systems.

Bill Cole

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