Your company has four offices. Each office is configured as an Active Directory site.

You have an Exchange Server 2013 organization that contains nine servers. The servers are configured as shown in the following table.

A user named User1 is in a distribution group named D_Sales. The mailbox of User1 is in a database that is active on MBX4 and is configured to use MBX5 as an expansion server.

You need to ensure that email sent to D_Sales from the Internet is received by CAS1, and then routed
from MBX1 to MBX4.

What should you do?