Christian Hammers <firstname.lastname@example.org> writes: > Hello > > On 2004-07-20 Russell Coker wrote: >> (host mail3av.westend.com[126.96.36.199] said: 450 <email@example.com>: >> Recipient address rejected: Greylisted for 300 seconds... (in reply to >> > RCPT TO command)) firstname.lastname@example.org >> >> 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 sounds suspiciously like (dangerously stupid) challenge-response systems. How does this differ?
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