Your Inbox folder contains an e-mail message that was bounced back to you ("Delivery Failure Report") but you do not recall ever sending the original message... Is someone using your e-mail account to send messages on your behalf? ...or is your computer infected with a virus that sends messages on your behalf without you knowing it?
In 99% of the cases the answer to this question is: neither of the above. The message that bounced back to you was most likely originally sent to some invalid e-mail address by a virus that uses Windows address book entries on the infected computer (host) to forge ("spoof") FROM header. Such virus sends itself as attachment inside an e-mail message to e-mail addresses that it finds in the compromised host's address book. Someone who has your e-mail address in his/her address book must have his/her computer infected with such a virus. Unfortunately, there is almost nothing you can do about it, since there could be hundreds of people who may have your e-mail address in their address book. Because viruses disguise the actual sender of the message, you will not be able to tell who the real sender of the message that bounced back to you was.