[Greylist-users] Whitelist entries and 4xx failure codes - Note to implementors

[Evan Harris]

After getting several whitelist submissions that I can confirm don't
actually need a whitelist entry (based on my sites logs), I think I have
narrowed down an issue/detail that may need attention from developers of
alternate implementations.

When doing the initial testing of my implementation, I tested the use of
several different SMTP codes, and 451 was found to be the least problematic
in that it caused the fewest number of problems with various sites.

Some implementations appear to be returning an SMTP error code 450 or other
4xx code rather than 451 which is what I used in relaydelay.  Some major
sites (MSN/Hotmail and others) trying to deliver mail appear to try several
times in a very short time period, and then bounce mail if they get a 450
error code rather than a 451.

The actual behavior varies by site, but since most sites that have undesired
behavior have this pattern, my guess is that it is because a 450 is
generally used for a mailbox lock failure, and is considered a failure that
should be able to be resolved within seconds, and thus the short retry
delay.  451 seems to be handled as a more generic temporary failure, and
seems to produce the desired result much more often.

So, if you're using or developing an implementation that uses an error other
than 451, you might want to check into changing, or at least testing
further.

Evan