The mailman proxy has been abandoned for a long time now, so no fixes there.  I have some sympathy to fixing PureProxy, but since the stdlib itself doesn't use it, not a lot :)

At some point we will start cleaning up old code (probably a while after 2.7 is dead) after going through a release-deprecation-cycle, but that time hasn't arrived yet.  smtpd itself will stay because the stdlib test suite uses it, but you are right that MailmanProxy can be removed, as well as PrueProxy if we don't fix it.

We are not in general going to fix bugs in it that don't impact the stdlib itself, but if Barry (or someone else) wants to review and accept the PureProxy PR that's fine with me.  I don't myself have time to review it, I'm afraid.