These are APB messages, Ozzy...
Check out KB-03281 "What Is - Unable to process the message"
http://www.blackberry.com/knowledgec...?nodeid=728206
Since it is just one user, most likely user's mail db is corrupted... perform maintenance on it... RIM has a KB recommending how to perform fixup, updall, compact etc..
KB-02207 "How To - Run Fixup, Updall, and Compact on a Domino database"
http://www.blackberry.com/knowledgec...?nodeid=736193