Which phone are you using? Samsung Android device?
Email is a bit of hit and miss on this watch, and because I use the Outlook mail app, the watch's email app is useless.
Anyway, if you want to receive email's through the watch's app, you'll need to setup your email account(s) on a Samsung phone using Samsung’s default email app. If you use any other email app, the email app on the watch itself won’t work as it currently only reads from Samsung’s default mail app on a Samsung phone.
As for mail notifications, you can get them from any mail app, but unless the account is setup on Samsung’s default app as described above, they will only show through the watch, NOT the watch’s mail app.
Speaking of "notifications", you also need to make sure you enable the appropriate app in the notifications panel on the galaxy watch app on your phone. To test if you’re getting those notifications send yourself a test email. If you still aren’t receiving notifications, try enabling "All" apps and test again.
One thing to remember is the watch’s OS (Tizen) is basically tied to Samsung’s phone and apps, so non-Samsung phones or apps may prove problematic with the watch. See my post
here.