We'll look at some of them and separate truth from fiction. There are no standards, or there are too many. Also, there are proprietary solutions that provide secure e-mail in a manner that is completely transparent to the end user. Standards compliance is nice, but it is not the issue. The issue is interoperability.
Proprietary solutions operate only with themselves.
However, this may be sufficient. Think about it. Interoperability is more easily achieved in a single organization. It is difficult for end users to use. While it may be true that the concept of digital signatures is difficult for many to understand, encryption of e-mail is not so difficult to deal with.