In the Generator.flatten() (Lib/email/generator.py), the code sets, among others, the instance attributes _EMPTY (correct spelling) and _encoded_EMTPY (misspelling). Further down in that class, _encoded_EMPTY (correct spelling) is set as a class attribute, and this correctly-spelt version appears to be used in the _handle_message_delivery_status() method. The BytesGenerator class inherits Generator and overrides the correctly-spelt _encoded_EMPTY class attribute.

It seems that both _EMPTY and the misspelt _encoded_EMTPY instance attributes are not used. Perhaps they should be removed. Or perhaps they are not doing the job they were intended for and there is a real bug.