One solution which is particularly useful where you have to supply an email address when you register for something but you are not sure if they can be trusted. Use an email address from a redirection account where you can have many throw away addresses. There are many services doing this, Sneakmail (see below) is one.

Once you have set up your account you can create a new address for each commercial organisation that you deal with. If you ever get spam on one of the addresses then you know who sold you out. You can just delete the address. This has the advantage of blocking further promotional material from the firm that sold or gave away your address to a third party.

www.sneakemail.com offer this as either a free or premium service. They have also introduced a new greylist facility which may be of added benefit in some cases.

Another sacrificial address that you could consider is a Hotmail account. Some people look down their nose at Hotmail, but they have recently improved their spam reject performance. They are also good at not showing mail-verification images and blocking viruses. I have had many viruses turn up at my various email addresses but Hotmail has never let one through. You do need a net passport to use hotmail but otherwise it is fine. I use my hotmail address openly in newsgroups and I only get a half a dozen or so spams per day on it which all go into the spam bucket. I also use this for commercial contacts that require an email address.