Hello, 

Currently this should be using the browsers built in spell checker. Are you seeing any issues with this? 

Aw, easy, I use Grammarly; this was a client question. 

This does bring up a privacy issue. Any browser spell checker probably has access to your Inbox or any online form. 

So, the revised Idea would be a "private spell checker"—one that you do not give access to a company, Ukraine, or another. 

Hello,

If you are looking for a spell checker in webmail, you will need to have a browser extension like Grammarly. There is no way to have a spell checker run in the webmail compose window without involving the browser in some way. I would suggest looking into spell checkers you trust. 

Otherwise, we would be looking at something closer to a web client like eM client or Outlook.