It would be nice if the composer window featured a spellchecker. Sonnet is one nice candidate, but there shouldn't be a hard dependency on KF5. I'm thinking about an optional plugin. Getting this into the CI and into the autobuilds for Windows is going to be a *lot* of fun :(.
Thank you for the bug report. As this report hasn't seen any changes in 5 years or more, we ask if you can please confirm that the issue still persists. If this bug is no longer persisting or relevant please change the status to resolved.
Trojitá is no longer maintained, please switch to a maintained alternative like https://apps.kde.org/kmail2/ Sorry for the inconveniences.