There have been several requests to add support for a grammar checking tool like Grammarly or LanguageTool (the latter is open source).
If we were to implement this in any fashion, we’d have to go with LanguageTool, which has a public API. If implemented as a core feature, this would need to be a Pro feature, since the API is not free. (There is a free version of the API, but it may be too limited to be practical.)
Grammarly does not have a public API.
This may alternatively be a good candidate for a plugin.
I would also be interested in seeing Grammarly supported in Thunderbird.
I use a Mac, and I’m surprised that many apps I would think would not support Grammarly do. For example, macOS Stickies and Notes, BBEdit, the Verizon Message+ app, and even when you edit a text field in 1Password. But it doesn’t work with other apps that you would think would work, such as Evernote.