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What is GreyListing (was: Re: Christian Hammers...)


On 2004-07-20 Russell Coker wrote:
> (host mail3av.westend.com[] said: 450 <ch@lathspell.de>: 
> Recipient address rejected: Greylisted for 300 seconds... (in reply to > RCPT TO command))  ch@lathspell.de
> Christian's mail server is broken.

Err, no. It's not a bug it's a feature :-) Called "greylisting".

In opposide to normal black- and white-listing here postfix has an additional policy daemon that checks if the tripel "sending ip, from, to" is already in the database and if not, reply with a 450 aka "temporary(!) failure" code and take note of it.
If it's a real mailserver and not a trojan-winXP-desktop then it will
try it again in a couple of minutes. If it does the above tripel will
be whitelisted for the next days/month/whatever.

This mechnism has reduced our Spam amount drastically even on a mail account that had already SpamAssassin active.
(It's installed at the ISP where I work and currently in beta testing.
The nearby university already uses it with great effort.



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