I'm working on completing the API changes for the email package for 3.4. This means I'm adding some more stuff to the current provisional API, which I will then aim to make non-provisional in 3.5.
I've made a complete proposal to the email-sig, and have gotten some positive feedback but no detailed reviews yet.
One part of the new API proposal is independent of the rest, and I've documented that part and implemented it. I'm posting the patch for that part. The patch includes some of the documentation for the rest, though not all of it...you can ignore all the doc changes in the new doc page except for the docs for the three new methods implemented by the patch for the purposes of reviewing this patch, though of course feedback on the (incomplete) rest is welcome.
I'm posting this now because this part of the enhancement can be reviewed independently of the other changes.
Note: I'm aware that MIMEMessage duplicates the name of a class in the mime subpackage. By the time I'm done that class will be deprecated (in 3.5). I haven't got that approved by the SIG yet, though, so it may change :)