Sure the comments can be updated.  Some of them elsewhere have been already.

The reason for the email library to have a validation function is that it has an 'SMTP' policy that is designed to produce valid SMTP messages when the message is serialized.  (It also has an HTTP policy that is designed to produce valid HTTP header blocks...though I'm sure there are bugs there as well).  These policies are relatively new, even newer than the send_message method of smtplib.

If this gets done it time it could be fixed in 3.5...Larry is planning the final non-security-fix 3.5 release some time after 3.6.0 final goes out the door, as is our tradition.  After that it could only go into what will very shortly be the maintenance release (3.6) and the next feature release (3.7).

To contribute, create a diff against the tip of the default branch and post it here.  (We will be switching to github "soon", but posting a patch here will always work).  See docs.python.org/devguide for more details on contributing.