I believe thats when the email site itself sends you spam.... such as.... If you had you would receive spam from them and it would appear to come from you..... may i suggest switching to a different email provider?
There are some email viruses that will send emails to everyone in your address book as though it was coming from you. Do you have yourself in your own address book for some reason? Maybe check with someone else in your address book to see if they are getting the same emails. I was getting these from someone else for a while on a nightly basis and he had no control over them being sent. The good news is that it seems to stop at some point, maybe once the original spam-virus is confident that nobody is clicking on the link to buy online viagra. It seems to have a lifespan of about a month though.
Don't use . Free account for your immediate family and friends only, then you will know anything else is just spam.