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Last I checked (which was a while ago), while the UI is in XUL / JavaScript, the bulk of the mail handling (POP/IMAP/SMTP, MIME parsing, etc.) was still in C++

by mook 4y ago

Last I checked (which was a while ago), while the UI is in XUL / JavaScript, the bulk of the mail handling (POP/IMAP/SMTP, MIME parsing, etc.) was still in C++ that was extremely complex and hard to port to JavaScript. There was an attempt to rebuild a mail client in JS for FirefoxOS (the mobile phone thing that is now mostly KaiOS), but that's not used in Thunderbird as far as I know.

jcranmer 4y ago
The backend stuff is being (slowly) ported to JS from C++. SMTP should be implemented in JS now, I don't know about the other parts.