For some reason the Samsung email app when receiving notifications on your phone sometimes you don't receive them on your watch when connected via Bluetooth to you phone it happens on galaxy watch and active 2 this never used to happen so not sure if it's an update to the app or its a software issue.
Possibly try wiping the cache partition on the phone and that should resolve the issue 😃. Just search how to wipe cache partition (phone name) and you should find what you are looking for 😉. Hope this helps! 😁