Reverse DNS (formerly IP Whitelabel) allows mailbox providers to verify the sender of an email by performing a reverse DNS lookup upon receipt of the emails you send.
Reverse DNS is available for dedicated IP addresses only.
When setting up reverse DNS, Twilio SendGrid will provide an A Record (address record) for you to add to your DNS records. The A Record maps your sending domain to a dedicated Twilio SendGrid IP address.
A Reverse DNS consists of a subdomain and domain that will be used to generate a reverse DNS record for a given IP address. Once Twilio SendGrid has verified that the appropriate A record for the IP address has been created, the appropriate reverse DNS record for the IP address is generated.
You can also manage your reverse DNS settings in the Sender Authentication setion of the Twilio SendGrid App.
For more about Reverse DNS, see "How to set up reverse DNS" in the Twilio SendGrid documentation.