Admin option to require encrypted sending

[willhowells]

I'd be interested in an admin option what would require encryption. If a user or the admin doesn't have suitable credentials, rather than their receiving unencrypted emails, the emails would not be sent.

[fabacab]

This seems like a good idea, but even more important for more users will be an option to require emails be signed rather than (only) encrypted. I think we can do both.