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I think really ill/strange is that kind of item _assignments_ do _add_ multiple.
If msg[field] = xy would just add-first/replace-frist , and only msg.add_xxxx/.append(field, xy) would add multiples that would be clear and understandable/readable. 
(The sophisticated check dictionary is unnecessary IMHO, I don't expect the class to be ever smart enough for a full RFC checklist.)
e.g. I remember a bug like
msg[field] = xy
if special_condition:
 msg[field] = abc # just wanted a alternative
Never ever expected a double header here!
"=" with adding behavior is absurd IMHO. Certainly doesn't allow readable code.