Thunderbird is a free email client and if you want to check grammar for emails, use the Grammar Checker Add-ons. It checks grammar using LanguageTool. You can download it from its manufacturer website.Note: Make sure to use the most recent version of the Grammar Checker Add-ons.
You can check your spelling in emails on Thunderbird. For this, carry out the step-by-step instructions given on this page.
Launch the Mozilla Thunderbird email client on your computer.
Navigate to Menu and select the Preferences option.