Note: This test cannot be done on the QA site as it requires a site where notifications are enabled and where users have working emails.
You need a course with a standard forum, a teacher and two students, student 1 and student 2

1. Log in as the teacher and access the forum. Ensure or verify that subscription is optional in Administration>Forum administration>Subscription mode.
2. Post a new thread called "Example one" with a test message.
3. Post a second new thread called "Example two" with a test message.
4. Log in as student 1 and click the toggle icon to the right of the Example one discussion to subscribe to it. Do not subscribe to Example two.
5. Log in as student 2 and click the toggle icon to the right of Example two discussion to subscribe to it. Do not subscribe to Example one.
6. Log in as the teacher and post replies to both Example one and Example two.
7. As student 1 go to your email inbox and verify you have the reply for Example one but NOT the reply for Example two.
8. As student 2 go to your email inbox and verify you have the reply for Example two but NOT the reply for Example one.
9. Log back in as student 1 and click the toggle icon of Example two to subscribe to it.
10. Log back in as student 2 and click to unsubscribe from Example 2.
11. As student 2 post replies to both Example one and Example 2 (making sure that the subscription box is not ticked).
12. As student 1, go to your email inbox and verify you have replies from both discussions, Example one and Example two.
13. As student 2, go to your email inbox and verify you have NOT received replies from either thread.