How do I delete e-mail addresses that are automatically appearing amongst the names on my contact list? 

They appear when I enter a letter of the alphabet in the "To" box. 

I can't delete them since they don't actually appear on my contact list. 

For example, if I want to send an e-mail to John Smith and John isn't one of my frequent contacts, when I type in the J for John, I get e-mail addresses that I don't recognize, and several of them are from my Craigslist communications.   

I'd rather they not be there and would like to stop them from being there. 

How do I do that?

Thank you!