Question : What is causing email to sporadically bounce back?

A client of mine has email that bounces back to him every once in a while. If he tries resending it, it'll usually go through within 10-15 minutes. It might happen twice in a day or it might only happen once ever 3 weeks. The message is always the same: "error 5.5.0 smtp; 554 message failed". Upon performing an internet search, I've found several error messages similar to this one but not one exactly like it. Does anyone have a clue as to what this means? He is the only person in the company having the issue. They have a replicating server and two caching servers. The replicating server, obviously, is running Exchange.

Answer : What is causing email to sporadically bounce back?

The key to resolving this issue, is to take note of the address that he is sending to when this happens, as I am sure you have but...554's are a bit general, but are commonly related to the destination server rejecting the mail because it is detected as spam and/or they have you black-listed. Do a quick check, if you haven't already to make sure that you are not black-listed. DNSRight.com or DNSStuff.com, or similar site has the tools to check your IP(s). If all checks fine, contact one of the locations he is having trouble getting mail to, to see why they are refusing it.