Thanks for the explanation David.

It would be really useful if this was prominently noted in smtpd, it would have saved me spending my morning fixing these things. There's also (AFAIK) nothing about this being deprecated in the docs.

More generally I understand the idea of deprecated code that doesn't get updated to conform to latest conventions, but surely if the code exists in the standard lib. it should at least work as it was originally intended or be removed?

Since the PR is done and passing surely it would be better to merge it than leave PureProxy completely broken.

By the way, aiosmtpd is not in good shape:
1. Its proxy handler is blocking https://github.com/aio-libs/aiosmtpd/blob/master/aiosmtpd/handlers.py#L119
2. Its proxy handler appears to be broken in some other way, I couldn't get it to forward emails which is why I resorted to using standard smtpd.
3. It's built in a very odd way, it appears to use threading not asyncio to run the main controller https://github.com/aio-libs/aiosmtpd/blob/master/aiosmtpd/controller.py#L54