We run a single Exchange 2003 Standard server. On two separate occasions we have had the following happen, both times with the same sender and recipients.

A user with an an email address in domain A sends a message to two others with email addresses in domain B. I must stress that this is all internal and on the same Exchange server. Both recipients were listed in the "to" field. The first listed recipient received the message, the other didn't.

Checking message tracking in ESM, as well as manually checking the server Application event log, indicates that both messages were delivered without a hitch. However, the message just cannot be found in the second recipient's mailbox.

Using both Outlook and OWA I have searched all the recipient's Outlook folders, in case it accidentally got moved to another folder or was deleted. I also checked the deleted message recovery folder. No sign of it anywhere.

I'm completely baffled. How could a message that the system insists was delivered not actually be in the recipient's mailbox? What else can I do to try and track it down?