1) Configure server mail, and set admin mail to a real email address
2) Add a category to messageboard portlet
3) Click the subscribe button at the top right hand corner of message boards portlet. Not the actions -> subscribe, but just the subscribe button.
4) Add a thread to the board, and reply to it.
5) Add a thread under the category, and reply to it.
6) No email notifications are sent.
7) Click actions -> subscribe to a thread
8) reply to the thread
9) You receive an email.
Basically, you should receive an email in step 6, but you don't.
Root-level email notifications aren't being sent – this affects 6.0 only because root-level message board elements didn't exist until 6.0
Split from http://issues.liferay.com/browse/LPS-11798