You getting e-mail that looks something like this but you know you did not send:
"Mail delivery failed"
"Returned mail: user unknown"
"Returned due to virus..."
"It seems that you have sent an attachment. This account does not accept attachments."
There may be a few reasons why you may be getting this.
You have a virus on your computer that is sending out e-mail with your account.
Spammers are forging the "From:" field with your e-mail address. So when the e-mail reaches an unkown recipient, it gets returned to you.
Somebody that you know may have a computer that has been infected with a virus. The virus tries to spread itself by sending itself to other people by forging the "From:" address using an e-mail address that's in the infected system's address book (which may be your e-mail address).
Make sure your anti-virus software is up-to-date by downloading any updates.
Run a complete scan on your computer to see if this finds any viruses. You may still get returned e-mails until all mail servers on the internet have returned these e-mails.
Unfotunately, it is extremely difficult to prevent a bounced e-mail from coming into your Inbox because the bounced e-mail is a legitmate notice that the intended recipient does not exist; we recommend to simply delete those e-mails.
You may want to contact everyone who has your e-mail address in their computer's address book and suggest that they might want to check their systems for viruses. However, this may not be the problem and usually only recommended if the issue does not stop.
Summary: Generally the e-mails will stop in time, in about a day or so. So the best recommendation is to give it some time and they will stop as the spammers move on to another e-mail address.
How did they get my email address?
This does not mean in anyway they are using your actual e-mail account, have gained access or hacked access into your account. They simply are using your e-mail address as their return address.
Spammers have many techniques that they use to harvest e-mail addresses with range from buying a list of e-mail addresses from other spammers, to executing "dictionary" attacks where they send an e-mail to email@example.com, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org, etc. Discussion of the different techniques are outside the scope of this document, but more information can be found using a search engine like Google, Yahoo! or MSN