I received a bounced message for an email that I did not send, or I received a complaint from someone saying I sent a virus, but I didn’t send the email
Email messages can be forged. An email sender can simply change the “From:” field in their email program, or in more advanced cases, forge addresses through the SMTP protocol, inserting any email address in the “MAIL FROM:” section of the message.
Spammers often harvest email addresses by spidering web sites or forums, or through viruses which can harvest an infected computer’s contact lists. They use these harvested address in the “From” and “Reply-to” fields of their outgoing messages to avoid dealing with the complaints generated by the spam.