This is a video demonstrating what happens with Windows Server 2012 and Exchange 2013 when you load up over 2 million users into it. The server still runs ok and you can send emails to it, so the backend can handle the number of users, but it is the frontend that has not been designed properly when it comes to trying to actually administer the users. It also takes way too long to add the users as well, so while Exchange 2013 and Windows Server 2012 might claim to be enterprise ready, they are certainly not countrywide ready, so it looks like you will have to look for another solution if you want to administer the emails for a country that you might run.
The system was run from a virtual machine in Qemu, with 32GB of memory and it was run from a RAID0 2TB volume, so it had plenty of hardware thrown at it, but this did not seem to make any difference in the speed of the frontend.