Very slow email delivery (two months!)

One of our staff has just contacted me to complain that an email he sent (from Outlook 2007) on November 17th last year (via our Exchange Server 2003 SP2) has only just been delivered (in January).

A search with the Exchange message tracker shows that the message was indeed sent by our Exchange Server on Sunday morning to our email scanning service for onward delivery, yet the message carries a message header of November 17th.  The message delivery corresponds to the machine hosting our Exchange Server being restarted after a power outage on Saturday night - I'm 99% certain this would have been the first restart sone the message was sent on November 17th.  As far as I can ascertain, I think this is a one-off - I've not (yet) received any other complaints anyway.  I've attached a screen shot of the message tracker.

I'm under pressure to explain what could have happened to this message and to ensure that it does not happen again - this particular email was relatively trivial but if it had happened to something more important....  Can anybody offer any advice on where to start looking for an explanation/resolution?  I'm no Exchange expert but I like to think I'm relatively knowledgeable on the subject and I've not seen this behaviour before.
Message-Tracker-2.JPG