Hi Arun,
I believe the e-mail notifications being triggered when any user creates/likes/replies to a post (in Forum) is the OOTB behavior of AEM Community, provided the user has configured to follow notifications/activities. We observe this behavior and is working fine.
However, I need to customize the OOTB behavior to send e-mail notifications on certain conditions as already mentioned.
I am new to AEM, please guide how to proceed.