Two things to check in the return address:

1. the time, as Jim mentions
2. Is it the name of a person known to you, but the rest of the address is wrong?

If the second one is the case, and the message has an attachment with a generic name "kids.jpg" etc, its probably the Bugbear virus.

It fiddles the from address so if you follow your instinct of clicking on "Reply" and asking if it's a legit message, it won't get to the person whose name is on it.

One annoying thing is that people who get it will think you sent them a virus, but only the ones that double check the email address, or phone you, will get through to you.

The other thing, though, is that if you are infected with any virus that raids your address book and sends itself off to people, well if you are like me you have a lot of outdated addresses in your email address book. Those are the ones that generate the bounce messages you describe at the beginning of the thread. So check out your own machine for those nasties.