There's Mail client from appfour but it requires phone companion app and doesn't work "standalone". Having an option to occasionally check emails from watch (with cellular module) is the primary reason for this modification (and leaving phone powered off or at home).
It's still rough around the edges (some features may crash/not work or be hard to reach) but this is the first version and it allows me to read emails (tested with Gmail, was able to send email as well; on Huawei Watch 2). Newsletters obviously won't work as only the text part of emails is rendered.
Let me know what you think. I do not plan on any major UI rework but I can fix thing or two (if there are some fellow watch only users ).
TIP: Consider using "adb shell input text" for entering complex strings during account setup.