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There’s no explicit toolbar button to toggle text direction in the composer. However, Mailspring does automatically detect RTL text — when you type in a right-to-left language (Hebrew, Arabic, etc.), the paragraph direction is set to RTL automatically, and the correct dir="rtl" attribute is added when the email is sent. If you’re writing a mixed-direction email and the auto-detection isn’t working as expected, that would be a good feature request to file on GitHub.