Alias CNAME Record, In this post, I can use this method to add an Alias canonical name (CNAME) reserve record for your Web server to a zone in Domain Name Server on your domain controller.
Add Alias CNAME Record
1- Expand Forward Lookup Zones and right-click on your external domain (xpertstec.com) and then select New Alias (CNAME).
2- within the Name, field A name (autodiscover) for your certificate and then click on browse.
3- Select your DNS server > Forward lookup zones > your external domain (xpertstec.com) > so select mail and click ok.
4- Click ok.
For more details click here