For those that might want to tweak new reply notifications they receive via e-mail, you can change the setting via viewing your profile while in the forum (Profile link at the top of the page), then selecting the 'Notifications' link under the 'Modify Profile' menu.
The default settings will send you an e-mail alert only on the first unread reply. If you want to receive an e-mail alert for EVERY reply to a message you posted or replied to, un-check the 'Receive reply notification only for the first unread reply' check box. There are other settings there as well that control your notifications which are mostly self explanatory. If you have any questions, feel free to reply to this message.
Now, for those that REALLY want to spam themselves, you can be notified of ALL new posted in a given board. When viewing a board (the list of discussions in a board), near the upper right hand corner, there is a 'notify' button you can click, which will allow you to be notified of all new messages in the board your are currently viewing.
