I think this is a spammer technique. Probably Rubrick, Max or musick would be able to say more.

Four things:

1. Don't subscribe to mailing lists
2. When you forward on mail - make sure to delete all the forwarded addresses contained in the body. Also BCC names rather than CC or include them in your To: line
3. Filter
4. Dont' reply to the mail or click on the unsubscribe link You will be spammed for life.

Finally, contacting Yahoo! MSN or the other web mail companies won't make any difference. They are giving you a free email so why should they do anything else for you?

If you really want to avoid spam then use a Unix based mailer instead of a web-based one. Get Pine or Eudora or else install Linux OS and use their platform. You can then get an easy to install spam filter which will include black and white lists which shoudl filter out most of your unwanted mail. I get about five spams a day into my in box but my black list folder collects about 250 a day. You can't stop the crap coming to you but at least you can control where it goes.