I am not getting notifications either, i signed in as Host...i am seeing exceptions but nothing email related.

Everything else works for email notification but my forums, i am using the latest version 3.20.9 and running DNN 4.5.1.

Anyone know how to fix this?

I am having exactly the same problem. All other emails work except emails associated with the forum. However, I realise that a new beta version is developed and has been sent for core integration. May be the problem is solved there. I am relatively new to DNN so I don't have any idea on how long this test takes. Is it matter of a week, couple of weeks , 2 month or 1 year. I know the standard answer is probably "it is done when it is done". However, I appreciate if someone can suggest how the schedule may look like based on past experience.

If the new version is coming out soon, then I guess we can wait for it rather than spening a lot of effort to resolve the issue in the current version.

 Thanks