This question popped up in my referrer log. The fact that someone found their way to my page indicates they didn't find a good answer elsewhere. That is probably because they were using Altavista to search. Who knew anybody does anymore. The Google Search is actually very helpful.
The short answer is: If somebody is using fake sender addresses in their spam messages so that it looks like you're doing the spamming, there's little you can do. Faking email is easy. There was supposed to be this big push for everybody to adopt something like SPF - where DNS is used to announce authorized email servers for your domain - but that hasn't happened yet.
It does make sense for you to publish SPF records for your own domain if your DNS provider supports it, though. Then, at least, when people complain to you about how you're spamming them, you can tell them that it isn't you - and if they were using SPF they wouldn't see the emails posing as you. GMail, among many others do use SPF records in spam scoring.