[wplug-internet] Postfix greylisting

[Pat Barron]

Every once in a while, I'll try to send a message to one of the WPLUG 
mailing list, and it will fail temporarily with an error like this:

Deferred: 450 4.2.0 <wplug at wplug.org>: Recipient address rejected: 
Greylisted,
see http://postgrey.schweikert.ch/help/wplug.org.html

Usually, if I just manually re-run the queue (sendmail -q -v), or just 
wait for it to happen automatically after the queue run interval passes, 
it'll succeed the second time.

Can anyone explain this behavior?  I'm just not familiar with postgrey 
or how it works.

--Pat.