In fact, the spammers can send messages that pretend to come fromanybody — that’s one of the tricks in the phishers’ toolbag, allowing them to forge messages “from” your bank. Not only that, but spammers can send email to your friends and business associates, while pretending to be you. Not good at all.

http://blogs.computerworld.com/15112/ask_amir_how_do_spammers_send_from_my_email_address