To check your emails, install Grammarly for Windows or Grammarly for Mac. Grammarly can help you write mistake-free nearly everywhere you compose your letters. This includes a variety of web clients (Gmail, AOL, Yahoo), as well as most desktop apps (Apple Mail, MS Outlook).
Note: Grammarly for Windows and Mac supports Windows 10 and all macOS versions starting from Sierra (10.12). It will not run on older OS versions.
You can also copy the text of your email and check it in the online Grammarly Editor and then copy and paste it back to your email.