In general, this draft is clear and well written.

I have 2 comments as below:
1. For Section 6, is it required to protect the UA capabilities negotiation messages? Have you considered the possible downgrade attack, like: the adversary tampered the UA capabilities negotiation message to make the UA work without s/mime? 

2. For Section 12, is there any general way to defend against the attacks from malicious or compromised intermediaries mentioned in this section? Maybe some guidances are helpful here.