1. Open Exchange Management Console.
2. Expand the tree of Microsoft Exchange On-Premises > Recipient Configuration
3. Under Recipient Configuration, right click on Mailbox and select New Mailbox (as shown below).
4. Introduction windows will appear. Choose User Mailbox and click Next button to continue.
5. At User Type screen, choose New user and click Next button.
6. At User Information screen, fill up the user and account information. After finish, proceed with clicking on Next button.
7. For next step, you can specify the mailbox, retention policy and Exchange ActiveSync mailbox policy. If not applicable, just leave default and click Next button.
8. Click Next.
9. Click New.
10. Finally, click Finish.