I used some of this code in writing tests for the auth_login failure in issue 25446 (thanks, Milan, you saved me a bunch of work :)

Now that we have asyncio and asynchat is deprecated, we've decided that the only real purpose of smtpd going forward is the smtplib tests.  Any other smtpd use should be converted to aiosmtpd instead.  Perhaps that library will eventually make its way into the stdlib and test_smtplib can use it as well.

So, I'm going to close this issue, but we may want to consider if and how auth fits into aiosmtpd (maybe as part of a 'testing server').