Mailboxes are provisioned by assigning the corresponding license to the user. Anything containing an Exchange Online license would do.
Trying to provision outlook email and calendar to active directory users

Iwork
21
Reputation points
New to active directory, using it to prototype a third-party tool that uses it to SSO users. I created multiple test users who have usernames such as <user>@<org>.onmicrosoft.com but when i try to sign into the accounts via outlook i see this message "A mailbox couldn't be found for <user>@<org>.onmicrosoft.com". Any help as to how i can create these mailboxes for my test users?
Accepted answer
Well you can proceed by... purchasing a license :) Or signing up for a free trial of Office 365: https://go.microsoft.com/fwlink/p/?LinkID=403802&culture=en-US&country=US
Azure/Azure AD subscriptions on their own do not include this.
That helped!