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Description
Currently aiosmtpd uses email.message.Message to represent a received email. Since version 3.6 (I believe) the preferred class according to python documentation is email.message.EmailMessage.
Making this change would bring aiosmtpd in line with the supported versions of python (currently >= 3.7). It would, however, be a breaking change.
My quick look at the code suggests that the only changes needed to accomplish this would be modifications to:
handlers.Messagehandlers.Mailbox