How To: Remove 'Create an Exchange Mailbox' Page from New User Wizard
If Exchange is detected in an environment, the New User wizard will always prompt to create a mailbox. You may want to remove this page from the New User wizard for specific locations in Active Directory; service accounts, for example.
Create a new PowerShell Script Module with the attached script.
Create a new Provisioning Policy object, selecting 'Script Execution' policy and assigning the script module created in the previous step.
Link the new Provisioning Policy object to any OU that you want to remove the Create Mailbox page.