I don't understand "After running the recommended patch of Internet Properties - Advanced - International (in Windows 7 Professional) to set the UTF-8 query strings to the ON state, ...". Yahoo messed up in two ways. They have messed up windows-1252 encoded messages **all** the time. And then there were times were even UTF-8 encoded messages were messed up. You can work around the former problem by always sending in UTF-8: In Thunderbird: Tools > Options, Display, General tab, Advances button, Fonts & Encodings, Text Encoding, Outgoing Mail. If UTF-8 is also messed up, you can force TB to sending QP encoded by setting pref mail.strictly_mime to true: Tools > Options, Advanced, General tab, Config Editor, paste "mail.strictly_mime". That's mostly written into the "user story" of the bug.
Bug 1435903 Comment 35 Edit History
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I don't understand "After running the recommended patch of Internet Properties - Advanced - International (in Windows 7 Professional) to set the UTF-8 query strings to the ON state, ...". Yahoo messed up in two ways. They have messed up windows-1252 encoded messages **all** the time. And then there were times were even UTF-8 encoded messages were messed up. You can work around the former problem by always sending in UTF-8: In Thunderbird: Tools > Options, Display, General tab, Advances button, Fonts & Encodings, Text Encoding, Outgoing Mail. EDIT: Correction: Tools > Options, Display, Formatting tab, Advanced button, Fonts & Encodings, Text Encoding, Outgoing Mail. If UTF-8 is also messed up, you can force TB to sending QP encoded by setting pref mail.strictly_mime to true: Tools > Options, Advanced, General tab, Config Editor, paste "mail.strictly_mime". That's mostly written into the "user story" of the bug.