If a user account has already been created manually through Active Directory and a mailbox is required for the user you will need to run the following PowerShell command for Exchange to create the mailbox.
This process is also called “mailbox-enabling”
For this example we are creating a mailbox for the user account joe.bloggs which has the Active Directory “Full Name” of Joe Bloggs.
Enable-Mailbox -Identity "Joe Bloggs" -Database "Mailbox Database"