When you signed up for the mail service did you uncheck the box that says something to the effect of "uncheck this box if you do not wish to be contacted my 3rd party companies regarding services or promotions"
It is worth checking out your account details. The reason why i say this is that i was in the same predicament the other day and i looked on my account details and to be able to use mail forwarding for outlook express i had to leave the box checked which i was annoyed at.
The other thing is...is spyware. Spyware can log such as things as Email addresses you have used or even keystrokes. Might be worth you downloading a free Spyware detector and remover.