You can allow users to subscribe to selected topics. When a new comment is posted on the topic, subscribers will receive an email.
Enable this feature by going to Settings > Discussion Board > Followers and checking the ‘Enable topic following’ option. With topic following enabled, users will see a checkbox on each topic page allowing them to subscribe or unsubscribe to that topic.
You can change the wording next to the subscribe checkbox by editing the ‘Follow message’ field.
You can opt to display the number of users following a topic by selecting the ‘Enable follower numbers’ checkbox. You can amend the wording that appears after the number of followers by editing the ‘After followers text’ field.
With topic following enabled, you can choose to display which topics users are following on their profile page. In the Discussion Board settings, click on the Profile tab and select the ‘Following’ checkbox in the ‘Profiles tab’ field. When a user views their profile page they will see a list of the topics they are following.
You can also choose to make a user’s followed topics visible to everyone by selecting the ‘Public followed topics’ checkbox on the Followers tab. Anyone viewing a user’s profile page will see which topics they are following.