In one of my Yahoo accounts I've received a bulk email dated Feb 20, 2001.
Quite apart from the age of it, and the question of where it's been hiding for six years, I'm confused because I wasn't even hooked up to email back then.
As they say in the classics: WTF???

It is possible to send emails with a different date and is a method used by many spammers these days. For example if someone sends you a SPAM email selling Viagra then they would tend to set the sent date in the future as say 12th Spetember 2009 so that your mail software places it at the top of your list of unread emails due to it appearing to be the newest email.

Not saying the email you received is SPAM but that is something which is very possible to do.

Thanks, DAL. It was spam by the way, some online casino.
Not today or any other day, thanks!