Hi Jorhett,
I believe you have a problem that is not solvable by us. I suggest you open a ticket with Apple support directly. And if you already have opened up a ticket with Apple, then I suggest you follow up on your open ticket. Also, if you want to be of additioanl help please provide your pcap and logs.
Here's a few ways for you to create logs that are helpful for Apple:
Enable HTTP Activity logging
defaults write -g LogHTTPActivity YES
Start Mac Mail is EWS Connection logging mode:
Applications/Mail.app/Contents/MacOS/Mail -LogHTTPActivity YES -LogEWSAutodiscoveryActivity YES >& ~/Desktop/EWSConnectionLog.txt &
Do not post this information here, especially the EWS Connection logging as it will contain very sensitve data.