An authenticated domain allows you to remove the "via" or "sent on behalf of" message that your recipients see when they read your emails. Authenticating a domain allows you to replace sendgrid.net with your personal sending domain. You will be required to create a subdomain so that SendGrid can generate the DNS records which you must give to your host provider. If you choose to use Automated Security, SendGrid will provide you with 3 CNAME records. If you turn Automated Security off, you will get 2 TXT records and 1 MX record.
Domain Authentication was formerly called "Domain Whitelabel".
For more information, please see How to set up domain authentication.
Each user may have a maximum of 3,000 authenticated domains and 3,000 link brandings. This limit is at the user level, meaning each Subuser belonging to a parent account may have its own 3,000 authenticated domains and 3,000 link brandings.