Editing Anti-spam techniques (section)

===PTR/reverse DNS checks===
{{Further|Reverse DNS lookup|Forward-confirmed reverse DNS}}

The PTR DNS records in the reverse DNS can be used for a number of things, including:
* Most email [[mail transfer agent]]s (mail servers) use a [[forward-confirmed reverse DNS]] (FCrDNS) verification and if there is a valid domain name, put it into the "Received:" trace header field.
* Some email mail transfer agents will perform FCrDNS verification on the domain name given in the SMTP HELO and EHLO commands. See [[#Strict enforcement of RFC standards]] §{{nbsp}}HELO/EHLO .
* 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 of email that originates from these computers is spam, many mail servers also refuse email with missing or "generic" rDNS names.<ref>{{cite web | url = http://www.spamhaus.org/faq/answers.lasso?section=ISP%20Spam%20Issues#131 | title = Frequently Asked Questions | work = [[The Spamhaus Project]] | url-status = live | archive-url = https://web.archive.org/web/20070106065720/http://www.spamhaus.org/faq/answers.lasso?section=ISP%20Spam%20Issues#131 | archive-date = 2007-01-06 }}</ref>
* A Forward Confirmed reverse DNS verification can create a 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 reliant on the DNS infrastructure, which has known vulnerabilities, this authentication is strong enough that it can be used for [[whitelist]]ing purposes because [[Spam (electronic)|spammers]] and [[Phishing|phishers]] cannot usually bypass this verification when they use [[zombie computer]]s to forge the domains.