Setting up reverse DNS on an IP address allows mailbox providers to verify the sender when they do a reverse DNS lookup upon receipt of the emails you send. When you update your DNS provider with a DNS record provided by SMTP relay providers, and then send mail over your IP, the recipient’s email service provider performs a reverse DNS lookup (rDNS) using an A Record (address record). An A Record maps your domain to your IP address. When a mailbox provider looks up your A Record, they see your SMTP relay provider’s IP address. When they look at your IP address, they see the rDNS that match your A Record. This circular checking proves your SMTP relay IP association with your domain and your domain association with your SMTP relay IP.
https://sendgrid.com/docs/ui/account-and-settings/how-to-set-up-reverse-dns/