Email hacked

So i wake up this morning to pick up a few texts from friends who ask me why i was inclined to email them at 2am this morning, to inform them of a really cheap place to buy an Apple iphone.....

When i checked my email account, i had quite a few Delivery Status Notification updates, which looked as if i had tried to send this info to everyone in my mailbox. I didn't.
Has anyone else experienced this, and whats the best course of action to report it?
Cheers for now

Same thing happened to me twice now, a couple of years back and late last year.

Same kind of crap - sending out loads of emails to my contacts list pimping stuff like cheap iPods and Blackberrys.