MSR63 - seeing some of your replies, I can tell you know your way around IT.
That having been said, bear with me while I ask some questions which may insult your intelligence.
Do I gather correctly - the user is receiving mail on an email system you manage?
1. Were the messages local (to the email server) or did they traverse a bridgehead?
2. If they crossed a public network, is there header information which you reviewed?
3. Does your Internet Service Provider (web site host, DNS host, etc) have provision for Secondary Mail Hosting? If so, what is their retry time?
4. Did all the messages come from the same remote domain?
Thanks.