It is a simple procedure to specify the mail servers (in our case it is sdcsend.com) allowed to send mails for this domain. This is done by publishing the text records in DNS settings. This makes the receiving server to check the DNS records to find out whether it is authentic.
It is a variant of SPF and used specifically by Microsoft based email service providers such as Hotmail/ MSN and AOL. It also follows the same routine of entering the text record to the DNS settings. Learn more about Sender ID