The PTR DNS records in the reverse DNS can be used for a number of things, including:
- Most e-mail Mail Transfer Agents (server software) use a FCrDNS verification and if there is a valid domain name, put it into the "Received:" trace header field.
- Some e-mail Mail Transfer Agents will perform FCrDNS verification on the domain name given on the SMTP HELO and EHLO commands. This can violate RFC 2821 and so e-mail is usually not rejected by default.
- To check the domain names in the rDNS to see if they are likely from dial-up users, dynamically assigned addresses, or home-based broadband customers. Since the vast majority, but by no means all, of e-mail that originates from these computers is spam, many mail servers also refuse e-mail with missing or "generic" rDNS names.
- A Forward Confirmed reverse DNS (FCrDNS) verification can create a weak form of authentication that there is a valid relationship between the owner of a domain name and the owner of the network that has been given an IP address. While weak, this authentication is strong enough that it can be used for whitelisting purposes because spammers and phishers cannot usually bypass this verification when they use zombie computers to forge the domains.
