I would like to see some visual feedback that an outgoing email is going to be encrypted. For example by making the background of Subject green (or configurable color), or perhaps all headers green (To + additional ones + Subject:).
Thanks for reading,
Or show it in a preview window, to be enabled/disabled in preferences as "Show preview":
"This is what your message source will look like: Send now ? ok / cancel"
I second this request. See the attached image for an example of what the UI could look like.
Especially if people normally communicate over encrypted email, a red glow on unsecured messages shouldn't be distracting.
RT also does this (https://www.bestpractical.com/release-notes/rt/4.0.12), giving a red background to forms whose reply goes to the customer, to prevent accidents.