What is Reverse DNS?
Reverse DNS simply allows us to verify that the sender of a mail is in fact valid. In and of itself it is a simple front end check that at present is blocking some 2.8 million emails every day.
This simple check also catches some email from legitimate sources BUT these sources have incorrectly configured mail systems.
Most commonly affected are people running their own mail servers off the back of ADSL lines or leased lines. The fix is completely simple – all the sending mail server needs to do is get their rDNS setup by their ISP.
Takes 24 hours and all good ISP’s will do it free of charge. As for ISP’s using the rDNS validation they are many and include BT, AOL, Virgin, 1and1, Fasthosts and so on.
As for a definition:
What Is Reverse DNS?
Reverse DNS is a way of associating an IP address with its domain name.
The reverse DNS identifier is contained in the PTR portion of the IP Zone File.
The IP Zone File contains all the different ways that your IP and domain name can be associated; each association serves a different need.
Iomart requires that all connecting Mail Transfer Agents have established reverse DNS, regardless of whether it matches the domain.
Reverse DNS must be in the form of a fully-qualified domain name. Reverse DNS containing in-addr.arpa are not acceptable, as these are merely placeholders for a valid PTR record. Reverse DNS consisting of IP addresses are also not acceptable, as they do not correctly establish the relationship between domain and IP address.
Reverse DNS that may be similar to dynamic IP space (containing pool, dhcp, dyn, etc.) may be treated as suspect. Therefore should be changed to reflect a fully-qualified domain name with standard MTA reverse DNS.
As for specific help:
Reverse DNS Bounce Messages
Reverse DNS works in the opposite way to a domain name. Where a domain name resolves the name to an IP address, reverse dns resolves the IP address to the domain name. We use this to prevent spam email being sent through our mail servers.
This works because typically a spammer will send mail from an invalid IP address, i.e., one that doesn't match the domain name that he is sending from. If the IP address doesn't match the record for the domain name, then the mail will bounce back saying that it is possibly forged. (see below)
"451 4.1.8 Possibly forged hostname for 80.XX.XX.XX"
Similarly, if there is no reverse dns setup then the mail will bounce with a message to fix reverse dns for your IP address (see below for example)
#550 5.7.1 Fix reverse DNS for 212.XX.XX.XX or use your ISP's mail server ##
Lastly you may also receive the following error which is quite rare and tends to occur when the IP Address can not be checked at all.
Client IP address 213.XX.XX.XX does not resolve
If you are experiencing problems with this, or people are unable to send you mail and are getting these error messages, then reverse dns will need to be enabled on the IP address that the mail is being sent from. This can be done by contacting the provider of the IP address (usually the Internet Service Provider) and asking for them to enable this. Another method of resolving this is to use your ISP's SMTP server to send mail. This can be obtained from contacting them for these settings.