This document is an ambitious attempt to add STS (strict transport security) to SMTP. It carefully deals with all the traps and pitfalls that were found in developing STS for HTTP, DANE, and so on. I believe that it has hit all the obvious security issues how a determined attacker might cause a downgrade; in so doing, it has become a very complex protocol. However, the authors make a good argument for each of the complexities, which is admirable. --Paul Hoffman