I just thought about myself and tried to edit the note using mutt. Since emails (via imap in any case) are not intended for editing (as @CodaFi is correctly indicated), mutt copies the note to the editor, saves it as a new "email address" and deletes the old one. An edited note syncs with Notes.app just fine.
Beware: the title / subject of the note / letter is deduced from the first line of the body.
PS (I do not think this affects the fairness of the above): My setup is a private IMAP server and mbsync with local Maildir and mutt.
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