Resource mailboxes that are created by using Exchange Administrative Center are not updated by address policy

The Automatically update email addresses based on the email address policy applied to this recipient option can be manually selected on the resource mailbox once the room is created. It will then be updated by the Email Address Policy. 

Administrators can use PowerShell to create the resource room mailbox, and can leave out the PrimarySMTPAddress parameter. This will allow the resource room to have the Automatically update email addresses based on the email address policy applied to this recipient option selected automatically.

For example, an administrator can run the cmdlet as follows:

New-Mailbox -UserPrincipalName confmbx@contoso.com -Alias confmbx -Name ConfRoomMailbox -Database "Mailbox Database 1" -OrganizationalUnit Users -Room -ResetPasswordOnNextLogon $true