You probably have a bug in your computer. Read the following link. http://www.webopedia.com/didyouknow/Internet/2004/virus.asp

Does it affect Mac OS X? Cause im running on a iMac.

Welcome to fluther @kaledia – your problem will have a solution in a jiffy.

Do you use a separate email program on your computer, or do you always use the web?
Either way, change your email passwords.

If you use separate desktop software, it could be a virus using your machine to send email.

If you always use webmail, and the mail is going to your contacts, someone hacked your account and grabbed the list.

Here’s a thread that you might find helpful.

When you use Yahoo Mail and most other ‘free’ services, you agree to let them use your conacts as they see fit. They will spam or anything else that adds to their ability to earn revenue from your activity.

@YARNLADY I do not think Yahoo, Gmail, Hotmail, etc send email to our contact lists. I could be wrong, but I have never heard that.

In this situation, forging the sender’s name is a US federal crime, legit companies aren’t doing that.

@jaytkay They not only send to your list, they even send from your contacts to you. Every day I get notices that (blank) and (blank) want to contact me, or even worse ”(So and So) is spying on you”. It’s totally disgusting, because some of the (blanks) have long since abandoned their accounts, and I would really like to get in touch with them. I know this because I have several accounts with contacts between them and the spam shows up very clearly.

@YARNLADY, it’s not “Yahoo!” doing the spamming.

I’ve just changed my password, hope it helps.