My thoughts on this:
Indicator whether the next message will be encypted.
Needs to come from the core and get live updates, so we can be sure it never sends unencrypted when displaying otherwise.
I suggest a small ingicator that has an info button or can otherwise be exapnded to show the reason.
Encrypted (verified chats)
All messages in verified chats must be encrypted
Has key of [user] and will send encrypted. Got the key verified contact / directly / gossiped from 
Unencrypted (no reply yet)
DC doesn’t have the key of this person yet, your messages will be unencrypted until they reply.
Unencrypted (reply, but no key)
Looks like your chatpartner hasn’t setup encryption.
Ask them to do so and tell them about autocrypt and deltachat, if they use plain email.
Can’t send encrypted: Key invalid [reason]