@Saumya Supekarwe leverage DM notifications heavily in our project.  Whenever DM stops sending an email that sometimes happen after new DM update or monthly maintenance, we often find exceptions in PDC (Pega Diagnostic Center) and were able to resolve the email error.  Some common errors were related to SMTP/network error (non Pega) that we had to resolve with client IT.  Sometimes client IT had to add new IP addresses (of the DM orchestrator nodes that were restarted during monthly maintenance) to their whitelist to pass along.  If you don't use PDC, you can check Pega logs (of DM orchestrator server) for any exception.  If you suspect a Pega product issue, then you can raise a Pega incident.