Creating users after mx records have been changed

Creating users after mx records have been changed

So during the setup phase I wanted to just make one user and then create the rest later on. However it said something to the extent of you can only create users before changing the mx records.

Am I right in assuming this means that after I change the mx records I can't create more users?

I don't see why that should be the case but want to double check before changing them.

tl;dr Can I setup additional users after changing my mx records?