The e-mail address from which spam and virus messages are sent is almost never what appears in the "From" line of the message header.  In fact, it's quite common for both viruses and spammers to use completely fictional e-mail addresses in the "From:" and "Reply-To:" fields of their e-mails.  Usually the "From" address is invalid, non-existent, but sometimes the message is sent using a real e-mail address that does not belong to the spammer. This can be an embarrassment to the real owner of the address, and can also confuse the recipient.

E-mail with forged headers can cause many problems. When the standards for Internet e-mail were finalized in the early 1980s, few programmers could have imagined how modern malcious virus software and "spammers" would misuse the service. They did not implement many security measures and as a result, the return address on an e-mail may bear no relationship whatsoever to the person who actually sent the e-mail.

Spurious Error Messages
Some spurious error messages may be sent to you as a result of spam or viruses which appear to have been sent by you. For example, the common error message reporting that e-mail could not be delivered is typically sent to the address that appears in the "From" line.  As a result, if your address has been used in spam or virus messages you may receive a number of error messages relating to messages which you never sent. In some cases the number of such spurious error messages can be quite large.

If you receive these spurious error messages, do not worry.  If the only indication of anything wrong is that you have received spurious error messages, then you should take no action.  Just delete the error message and forget about it.