I'm not sure exactly what you are asking, but maybe this information will help:
To receive an email when a student makes a post on a forum, you must be subscribed to the forum. Click on the forum and then in the forum administration look for the link called "Subscription Mode."
Typically, a user has time to edit their post. The post is not delivered to your email until that time expires. However, if the user clicks on the "Mail Now" check box, the message will be delivered within about five to ten minutes.
If you go to your profile and clicking on "Messaging" you can choose which types of notifications to receive and the method by which you'd like to receive them.
You did not specify which version of Moodle you are using, but you should be able to do these things on most versions.