I've combed through the traffic logs of our firewall to get more details and it looks like the Outlook iOS App is connecting to IPs that do not seem to be in the official list. The thing is, every iOS client connects to a different IP, so excluding is kind of impossible.
iOS Client 01 - wants to connect to 184.108.40.206 via 443 and is detected as SSL
iOS Client 02 - wants to connect to 220.127.116.11 via 443 and is detected as SSL
As soon as I exclude a specific IP destination from SSL decryption/inspection this one specific iOS Outlook Client works flawlessly.
Currently experiencing this same issue behind Bluecoat Proxy with HTTPS intercept AND trusted proxy certificate on the client mobile devices. Also using Microsoft InTune to deploy trusted certificates. Was there a solution to this issue?