When you try to create a new mail server, under System > Incoming Mail, if you select the Custom Option, MS_Graph is available as Protocol, however the hostname can not be changed (for setting up a High GCC account):

If you change the Protocol for POP:

Switch the Service Provider for Microsoft Graph API (note that you can not change the hostname here as well, so it won't work for High GCC environments):

And switch back the Service Provider for Custom, you will be able to edit the hostname: