MTA status codes (visible via setdebuglevel(1)) are not accessible as the `sendmail`-method stores them only locally (code, resp).

I suggest to store the mta status codes, for instance: "250, b'2.0.0 Ok: queued as XYZ" etc. in an instance attribute (tuple) to access them on smtp-sessions. 

As an email developer those information are very important to me.

Thanks for the PR, but this is a duplicate of #29539, which I think has a better API proposal.  Since the original author never actually submitted a PR there, perhaps you could pick up his work (after pinging the issue).