Nowadays it is quite common to receive a message which talks about an e-mail coming from you that was either infected or undeliverable, but you are not aware of having sent it.

In 99% of the cases it is not you who sent the message and not even your computer. Rather, the real sender (which can be a person or a program, e.g. a computer virus) inserts an arbitrary e-mail address (which happens to be yours) as sender. There is nothing magic about this. It is not necessary to have any hacker knowledge or access privileges to do that.

So, why do virus scanners send these notifications when it is extremely unlikely that they get to the right person? The answer is that sometimes users mail documents of which they don't know that they are infected. In such a case it would not be appreciated if the message were silently deleted because it is generally expected that a sender gets notified of any delivery problem. For this reason you will have to live with the annoying virus notification messages. Simply disregard them if they talk about a message you have not send knowingly.