I like to use digital signing in email, and some friends have done the same and we can exchange fully encrypted email. However, it's a major issue that iOS has no support for this. If you get an encrypted email, this is all you can see.
There is nothing you can do with the email. This is a shame, since it would be really easy to get all this information from my keychain on my Mac.
Hope Apple decides to add support for encrypted email at some point soon.