Failed delivery of emails you never sent can be alarming, and for good reason. These notifications could indicate a spammer or virus using your email address. However, you can generally ignore them.

Spammers rarely send their unsolicited messages with their own email address in the From field. Not only would this reveal their identity, it would also allow recipients to write angry replies. (However, you can still track down where the spam email is coming from and file a complaint with the spammer's ISP.)

Authors of worms and viruses want the opposite of what spammers want, but the result is similar. Social engineering is essential to spread infections, and the key is to make the malicious code appear to come from a friend or trusted source.

At the same time, the From line should not contain the e-mail address of the owner of the infected computer. The response from a virus filter, telling them that their computer is infected, can warn them. That is why worms put real, but random addresses in the From line. They usually get them from the address books of the e-mail clients.