...
user won't receive any notifications about his own topics until he manually enables such setting:
Div 
- user, who added new post is getting e-mail about that, but this isn't very useful unless for spamming purposes
- users who participated in discussion (added posts to the topic in past) aren't notified about changes made to the topic
Solution
- implicitly enable "Notify me about replies" and "Notify me about Private Messages" settings (will be enabled unless user have manually disabled them)
- notify all participants in the discussion (except user have added a post)
- enable notifications for all existing topics in upgrade script
In-Portal Forums
Once ready I strongly recommend applying described above changes to the In-Portal Forums as well.
...
- user count: 5636
- user count, that have enabled notifications: 40 (0.7%)
- topic count: 4147
- topics with notifications enabled: 315 (7.6%)