When the user posts their message their is a box they can check if they want to be notified or not. The box says Check here to subscribe to this topic. You'll have to have email turned on in order for it to work. You'll also have needed to set the subcription level for the forum as well.
Also, they can click the Subscribe to this topic link at the top of any post.
If I want to change "Check here to subscribe to this topic" to "Notify me by email when somebody replies to this message", where I have to edit it ?Which File ?Post.asp ?...
Check this out and make sure you didn't miss a step somewhere.
Edit: I should probably add the standard caution that if you aren't careful how you pick and choose which forums generate subscription notices you can slow your site to a crawl.
Last Edit: It may also be worth looking up a MOD that was developed a while back (the name escapes me at the moment) that would only send one notice until the person actually logged back into the forum. It cuts down on the volume of e-mails sent so a person doesn't get 50 e-mails about the same topic.