[rrnkgssu] Email requiring whitelisted signature

There seems to be enough rope to implement email requiring public-key cryptographic signatures of identity.  The receiver rejects and bounces mail unless the message is signed with a key on the sender's whitelist.

The sender's client automatically signs messages to be sent.

Distribution of the trusted keys, a.k.a, PKI, is left unsolved.  Perhaps ad hoc, or a social network web-of-trust.