How do I stop illegal use of my domain email?

What you described is a common reality: at some point, somene might "spoof" (you can learn more via the link I'll include) your email address and send out emails believed to have come from you. It's difficult if not impossible to solve, especially if you have little knowledge how this occurs.

Unfortunately yahoo ISN'T going to help you fix it. They're as helpless as you are despite their resources, although they might post suggestions or something.

Bottom line: either you read up on the subject and try to fix it yourself, or get someone technically well-versed to handle this for you. One clue that might help stop this problem, though is to use an "email authentication" method like SPF

Spammers use what is called spoofing to fake sending addresses. Unfortunately there is not a lot you can do about it. If you can get a few messages that they have sent you can possibly find the server that the messages are being sent through and contact the owner of that machine. It's very difficult to prevent this sort of thing from happening.

Your domain might be Categorized as spam in a lot of email servers.

Try here they have live chat.

http://help.sbcglobal.net/article.php?item=10400