Fixed mine. I was getting all notifications on Watch except mail. Tried reset, restart, turning off cover to mute, etc. Finally, I realized Watch was set to Mirror my iPhone. I don't like receiving notifications on my iPhone lock screen, so I have those turned off on my phone. Apparently, that prevents the notification from coming through on Watch. To fix, do the following in Watch app on iPhone:
1. Navigate to My Watch > Notifications > Mail
2. Select "Custom" instead of "Mirror my iPhone"
3. Turn on "Show Alerts"
4. Tap each mail account and choose if you want to receive alerts, and if you do, select the type(s) you wish to receive.
Hope this helps.