Thunderbird has long missed out of a grammar checking library or addon. Companies like Gammarly have fallen over themselves to get addons for browsers, but none has shown interest in supporting desktop mail clients like Thunderbird.
This week that changed as the Grammar and Spell Checker - LanguageTool has been ported from Firefox to Thunderbird.
The tool has been available for Firefox for some time and has some 170,000 users on Firefox with almost 2000 five star reviews ad a Recommended badge from Mozilla so I was determined to try it. So far I like it.
The grammar and spell checker for Thunderbird is obviously very new and there is little to judge it by except the reviews for other products which appear to be mostly very good. I have only used it in Thunderbird for one day and it has so far been quite useful, except I have to unlearn the right click spelling for Thunderbird. Now it is right click.
The addon can be downloaded from here and I recommend it as it is the only game in town. But it does have some fairly good features and supports a wide range of programs and writing platforms in the paid version.
The developers web site https://languagetool.org/ indicates the developer hosts it's servers in Germany and that the product is GDPR-compliant so the privacy of data will be a hight priority.