Grammarly is a great tool at the right place and time. It does not work well within the HTML editors JangoMail uses, as it causes massive bloating of the content size and extremely long page loading times. We have disabled the integration between us and them, and our message content validation includes a check for Grammarly-related content. If you have any Grammarly items in your content, your message will not pass validation. Please remove Grammarly from your content and your message will validate (assuming no other errors).