This issue depends on issue 21725 for the communication with the client and issue 22027 for communicating with the remote server. I think I can come up with a patch for the default branch which is applicable but will not run without the two other issues beeing fixed (I tried that for issue 21783 but it didn't work out well).
I just wanted to note that this patch works with the current default repo as expected. I will try to come up with some tests now (PureProxy is not tested at all right now).
After looking at this patch and how we implemented SMTPUTF8 in the parent class again, I think we can come up with a more generic solution. Haven't it figured out quite yet.
I'm closing this as won't fix since smtpd.py is deprecated and will likely not get any future development. Please see aiosmtpd as a much better third party replacement.