Applescript para cambiar la línea de asunto

He estado buscando durante los dos últimos días y he encontrado unos cuantos applescripts que decían cambiar las líneas de asunto. Pero ninguno de ellos parece ser capaz de cambiar / establecer la línea de asunto. ¿Cuál creen ustedes que es el error aquí?

tell application "Mail"
    set theSubjectList to {}
    set themessages to selection as list
    repeat with msg in themessages
        set thisSubject to (subject of msg as string)
        if thisSubject is not in theSubjectList then
            set theSubjectList to theSubjectList & {thisSubject}
        end if
    end repeat

    set theNewSubject to ¬
        (choose from list theSubjectList with prompt ¬
            "Choose the subject of the merged thread…") as string

    if theNewSubject is in theSubjectList then
        repeat with msg in themessages
            set subject of msg to (theNewSubject)
        end repeat
    else
        display dialog "Action cancelled"
    end if
end tell

Me aparece el siguiente error

error "Mail got an error: Can’t set subject of message to \"CHANGED SUBJECT LINE\"." number -10006 from subject of message