I am the assigned ARTART reviewer for this document.  I'm very experienced in email, and have some experience in the use of encryption.  While I haven't read this entire document before, I've had in-person conversations with the authors about it a few times!

It's very well written, and does and excellent job of calling out future work and laying out the complete problem space.  In that way I think it's a very valuable document, and as an informative document it should be published.

I do have some concerns about the implementability (particularly, as already called out in another review, the "strip things which aren't secure enough when quoting for reply" which will likely make users feel there's something wrong with their client).  Will client authors refer to this document when designing things?  Maybe.  It's better than not having anything.

With my "ART Area" hat on, there are no conflicts here with other documents or with work happening in ART. 

I recommend publishing this document.