Rated 5 out of 5 stars
LanguageTool is a mediocre grammar checker. I can't say I recommend the paid license, but the free plan is a welcome step up from vanilla spell checkers.
This add-on is excellent, however. Very responsive, and well designed. Only limited by the capabilities of LanguageTool itself.
Note, that you should be comfortable trusting a third-party with your privacy before using this tool. You can self-host the LanguageTool service, but by default it will send all of your composed email contents through the LanguageTool public API.