WCollins,
I have a Browser based email account that I use for signing up for forums etc. so my main email account does not get bombed.
About six months ago, I had an email from a friend dated 1999. Turned out he had sent it that many years ago and it finally made it to my account. My guess is the company I use for Browser email had performed some maintenance on their system and it recovered the email and finally delivered it.
Take Care,
Richard
P.S. In your case, it could be either the sender's PC or the Mail Server itself had the wrong date when the email was sent.