"Should" and "do" are different, though.  I wouldn't be at all surprised to find legacy code that did look at the text string part of the message.  Particularly test code, which is a place that smtpd gets used commonly.

Which probably means that what we really need is a keyword argument to the SMTPServer constructor (analogous to data_size_limit) that indicates whether or not enhanced status codes are enabled, and have it be off by default.  If desired, we could then issue a warning that enhanced status codes will be enabled by default in 3.6.  That kind of process is our usual backward-compatibility dance.  In this particular instance we could go one step farther and add a deprecation warning in 3.6 that the ability to set enhanced status codes off will go away in 3.7.  If anybody remembers to carry through on it :)