For Silver and higher level accounts you are issued a brand new, sender neutral, dedicated IP address. As we are the owners of this IP it originally resolves back to us.
By completing the Whitelabel Wizard and adding the necessary DNS records to your domain, the IP address resolves back to you and effectively masks us in the sending process. Your emails appear to come directly from your Whitelabeled domain and your Click Tracking Links and Open Tracking Images lie under the same domain. Only one domain can be Whitelabeled to a given IP, Subusers are used to Whitelabel multiple sending domains and this does require purchasing additional IPs for $20 per IP per month. Although it is not required, SendGrid highly recommends Whitelabeling of all new accounts in order to help you get the best deliverability possible.
For specific questions about how to make the required Whitelabel DNS changes to your domain, please contact your website and/or DNS hosting provider.