On Wed, 20 Apr 2005 08:22:46 PDT, Bill Verity said:

> "The reason your mailserver is getting blocked is most likely that it does
> not properly handle 45x SMTP error codes, and does not retry delivery within a
> reasonable amount of time after receiving such a code."

I've seen sites that botch greylisting - they'll see the "first" attempt, remember
it for 45 minutes (for example), your site retries an hour later, and it's counted
as a new "first" attempt.  Lather, rinse, repeat.

So probably you need to either shorten (if it's long) the retry interval,
or lengthen it if it's too short...