The point is that if people send their mail encrypted it is encrypted both in transit and when it is stored on the mail server. So the third party can be anyone

The point is that if people send their mail encrypted it is encrypted both in transit and when it is stored on the mail server. So the third party can be anyone, and you don't need to specifically trust them. Also this isn't that big of an obstacle you make it to be. We're not living in 1995 anymore. A lot of mail clients have plugins or even have built-in support for encryption.