If you have a small Exchange infrastructure you will most likely benefit from using only one namespace, like reducing costs on SAN certificates.
1) In case you are trying to set your Exchange Server to use only one namespace (ex. mail.contoso.com) you first need to change the internal and external FQDN for all Virtual Directories. You can use the following link to do that. LINK
2) The second and final step is changing the FQDN on the “Default Frontend” receive connector.
If you get an error when you click “Save” go to “security” and deselect “Exchange Server authentication”. Now you will be able to save.
To verify this configuration you can telnet to your Exchange server and EHLO. The result should be your single namespace FQDN (mail.contoso.com).
You can also change the SMTP Banner that is displayed when you telnet to your Exchange server. Visit my post HERE.