A lot of people have not updated their forum profiles to include a valid email address. If you ask to be notified of a new reply to your post and your email address is not valid you obviously will not receive the email. You need to update this to receive notifications of posts to forums, new PM messages or to subscribe to a forum and receive a daily digest of new posts.
The server receives quite a high volume of emails to the address email@example.com which is the default. If you haven't set yours up yet, follow the Edit Profile link at the top of any forum page, click on the User Profile link and update your email address.