Gmail will alert and sometimes correct your grammatical errors

The days of sending grammatically questionable emails might be over. Or at least, you're getting extra help from Gmail. Similar to what Google has done with Docs, Gmail will let you know if you've made grammatical errors and underline these in blue. You can right-click those words to see the suggestions on how to fix them. And if you make "common spelling mistakes," then Gmail will automatically correct those for you. This new feature is coming first to G Suite accounts by mid-September. We don't know when it's coming to regular Gmail users, but you can expect this to make its way soon.

Source: Android Police