Certain BYOD email suites use https encapsulation of another protocol (also using TLS) to ensure that the data can go through firewalls that do MITM attacks on clients for security reasons. Bluecoat do this for example.
I believe they also certificate pin the tunneled protocol.
Well not easily, because once the outer TLS has been set up, you can‘t see the contents of the second TLS handshake. You could maybe deduce it via packet sizes and timings, but certainly not pretty easily.
I believe they also certificate pin the tunneled protocol.