I got a message that my email is undeliverable because of a virus, but I did not send the email.
Answer
Some viruses spread by searching your address book, instant messaging program, or local files for email addresses. The virus then chooses one of the harvested email addresses for the sender's address and another for the recipient. This can make it appear that another person's computer is infected when in fact it is virus-free. This is also called "Email Spoofing"
For example, John is using a computer that is infected. Both Mary and Peter have sent email to John in the past. When the virus is active, it finds the email addresses of Mary and Peter. It inserts Mary's email address into the "From" field of an infected message, then sends the infected message to Peter. Peter receives the message, which appears to have been sent by Mary. Peter then contacts Mary and complains that she sent him an infected message, but when Mary scans her computer, her antivirus program does not find anything (as would be expected) because her computer is not infected.
Currently, there is no way to prevent email spoofing. If you are using a good antivirus program with updated virus definitions, and no threats are detected by a full system scan, then it is unlikely that your computer is infected with any known virus.
Here is a partial list of common viruses that spread in this manner:
W32.Bugbear
W32.Klez
W32.Navarg
W32.Mimail
