Backdoor to Exchange server?
You can get excruciating detail from Mac mail client as follows:


open mail
select the window menu -> connection doctor
tick the log connection detail
note the wall clock time and select check again
open the log folder and look over the text logs that document exactly what mail servers are being queried, what messages are exchanged between the mac and the iOS device.


If the Mac can still connect, either your exchange setup has a director set up to point to the correct mail server (I believe Microsoft markets that as auto discovery) or it send a reconfiguration message to the mail client to update settings. Safari will also watch for you signing into web mail and offer to set up the mail client.

Best of luck taking those settings to iOS - you might be able to still connect after looking over the Mac connection details.