Official Thunderbird Grammar Checker add-on: https://addons.mozilla.org/thunderbird/addon/14781 Back-end functionality is based on LanguageTool functionality (in particular, the add-on uses its webserver). Though it can work without any further customization, it is strongly recommended to setup a local LT server (see below), and change the add-on preferences to use your server. By default, the add-on uses the local server and falls back to public LT server (that can't handle the high load and can't guarantee the privacy). The grammar checking is available via Tools menu, the context menu and the corresponding toolbar button. Language and server settings can be changed using the add-on preferences: Supported languages : English (en), Dutch (nl), French (fr), German (de), Italian (it), Polish (pl), Romanian (ro), Russian (ru), Slovak (sk), Slovenian (sl), Spanish (es), Swedish (sv), Ukrainian (uk), Asturian (ast), Belarusian (be), Breton (br), Catalan (ca), Chines...