Let's use this bug to track code that implements heuristics, that try to detect scenarios in which the user might need help.

If a user has never set up OpenPGP, we could encourage the user to do so. It's a good opportunity if the user replies to a message, and TB discovers that the sender (now recipient) has included information about their own s/mime or openpgp key.

Another example is a scenario, in which the local user has previously sent encrypted email to Alice, but in a new incoming email, a new public key for Alice is suggested.

We should have UI that makes the user aware of such unexpected events, and offer the user to interactively resolve it.

It might be useful to have statistical information about the past use of public keys of correspondents, for example, when a correspondent's key was seen for the first time, if it has been used before. While weaker than having manually verified ownership, such statistical information (as suggested in bug 1595235) might be useful.