[stomachmonkey]

Not saying this is the case but I've seen people do whacky stuff.

Change the date on their machine before they send a piece of mail, spammers are doing this lately by sending "mail from the future", makes it sit at the top of your inbox till the date is reached.

Or "backdating" so it sits buried in your in box, they insist they sent it "days ago" and when you look again it's there.

Or intentionally putting a typo in the To field. Later when the recipient complains they never got it the sender goes into sent mail and forwards the original "bad" mail. It will get populated with the original sent date and sender just corrects typo in the text.