Grammar, style, and spell checking (LanguageTool Plugin)
Feature Summary
Spell checking is already working great in Geary. But as there is a pretty powerful open source enhanced grammar, style and spell checker with LanguageTool (like Grammarly but with a free and self-hostable version and supporting many languages) which significantly improves language quality of E-Mails.
There are already plugins for Firefox, OpenOffice, Thunderbird etc.
How would you like it to work
I think a plugin would be a good starting point. If many users like the feature it could get a core feature in the future. I like the way it works in Firefox textarea-Tags (underlining errors with different colours and an on-click context pop-up with the explanation of the error.)
Relevant links, screenshots, etc.
https://languagetool.org/ https://addons.mozilla.org/en-US/firefox/addon/languagetool/ https://addons.thunderbird.net/en-US/thunderbird/addon/grammar-checker