If you find that your software needs special registry keys called GlitchRetrySeconds, well,
you might need to figure out if you should be writing smtp servers at all.
http://technet.microsoft.com/en-us/library/8b43be56-48e6-400b-8014-54c95f87d1de.aspx
It seems that if an Exchange 2003 server talks to a server employing blacklisting, it stands a
good chance of "forgetting" about your email until the server is restarted (or smtp is restarted).
Here is a hotfix and a better write up than I can stand to write now:
http://support.microsoft.com/default.aspx?scid=kb;EN-US;950757
This dude deserves a medal for not dropping 4 letter words (as I would): (p.s. Thanks
http://absoblogginlutely.net/2009/04/exchange-2003-ndrs-are-sent-a-long-time-after-email-was-sent/
Here is a more complete write-up:
http://duitwithsbs.wordpress.com/2008/06/15/exchange-2003-sp2-and-greylisting-michaels-meanderings/