Hello HarrisNo,
We would recommend ensuring both devices are updated with the latest watchOS and iOS versions. This will help ensure they have the latest available fixes and improvements. You can find the option to update to watchOS 8.5 and iOS 15.4 at Settings > General > Software Update on each device. Be sure to back up the iPhone ahead of time, to help preserve your data.
Update your iPhone, iPad, or iPod touch
Update your Apple Watch
When you note email isn't syncing, which aspect seems to be affected? For example, are you not receiving notifications or the emails themselves? Or does deleting an email on the Apple Watch not delete it on the iPhone?
The 'Mirror' option will be related to matching particular settings from the iPhone, rather than syncing content, just to note. You can try changing that to 'Custom' to review the settings in question.
A step that can help with issues like this would be to restart both the iPhone and the Apple Watch. If it still persists, you can try unpairing and re-pairing both devices:
Restart your Apple Watch
Unpair and erase your Apple Watch
Cheers.