1. Open Exchange Admin Center (EAC)
2. Click on “Servers” on the left hand side > Virtual directories
3. Click on “Configure external access domain”
4. Under Select the Client Access servers to use with the external address, click Add+
5. Select the servers that you want to configure and then click Add.
6. Click “OK”
7. Enter the external DNS hostname that you want to use for external e-mail access
