DNS is the opposite. It takes the IP address and queries the DNS to see what
friendly name that IP address is assigned to.
While REVDNS records are not absolutely required, virtually all
legitimate servers have them. Theoretically, the only computers
delivering mail to our network should be other legitimate mail servers.
Although many mail providers do not accept mail from servers lacking RDNS (or
providing a suspicious RDNS entry), we do not. We do, however, penalize
such things in our weighting system.