Pro FeaturesSpellcheck settings are in Preferences > General under Composing. Make sure Check messages for spelling is on.
Windows and Linux
Use the Spellcheck language dropdown to pick one of the available dictionaries. (System Default) follows your operating system's language. Dictionaries are downloaded the first time a language is selected, so it can take a minute before underlines appear. Only one language is checked at a time; switch the dropdown when you write in another language.
macOS
Mailspring uses the built-in macOS spellchecker, so there's no language dropdown. Set your languages in System Settings > Keyboard > Text Input > Spelling — choose Automatic by Language to have macOS detect the language you're typing in.
Nothing being underlined on Windows? Update to the latest version of Mailspring — spellcheck in the rich-text composer was rewritten in early 2026 after a long-standing Electron bug.
See also: Using spellcheck