How to ensure users added to Exchange server 1, fixes a different domain address than when adding users to Exchange server 2

hi guys

We have a forest fc.local and different subdomains like uk.fc.local, us.fc.local and hk.fc.local. Seeing as the Exchange environment is intertwined, how can I create a user mailbox on the us.fc.local domain and ensure it is given a different default domain address than if I create a mailbox on the uk.fc.local domain?

So user A on the us.fc.local with contoso-usa.com address and user B on the uk.fc.local being given a contoso.co.uk default address?

Thanks
Yashy