Email notifications

You can remove email notifications altogether in the watch app on your phone> Notifications > Mail> Custom > set to off.

Or: How to mute or turn off an app's notifications on your Apple Watch

1. Touch and hold the top of the watch face. 
2. Wait for Notification Center to show, then swipe down.
3. Swipe left on the notification that you want to mute or turn off, then tap the More button .
4. Choose from the options that are available for that app. Depending on the app, you can mute notifications for the rest of the day, mute them for an hour, or turn them off completely.

You can turn on an app's notifications if you want to see them again:

1. Open the Apple Watch app on your iPhone, then tap the My Watch tab. 
2. Tap Notifications.
3. Scroll down to the list of installed apps, then turn on the app that you want to receive notifications from.

For WatchOS 9 see: Notifications on your Apple Watch - Apple Support

Did you try the instructions I posted below the screenshot? I can’t really look in my settings right now as I’m updating my watch, if you have an update available for it, install that. Otherwise you could try restarting your watch. You can also use silent mode but that would silence everything

Another article to check out: Change the audio and notification settings on your Apple Watch

You’re welcome