Monday, October 09, 2006

Mail: Reverse DNS and working with your ISP

In a recent post, I talked about how some mail hosts require that a FQDN be included in a reverse DNS lookup. A commenter noted that they were having some trouble getting their ISP to respond to requests for configuring a PTR record. So what I thought I’d do is go into a little bit more detail on how reverse DNS works, and then explain the process I use for finding the right person to make this happen.

Reverse DNS is the process used to determine what hostname is associated with an IP address. So that’s the reverse of a typical DNS lookup – which you would use to try and find the IP address associated with a particular host name. DNSStuff has a really good article that explains this if you want to dig a bit deeper.

So in order to implement reverse DNS lookup for an address, you must setup a PTR record for the hostname. Your ISP should have a PTR record already configured, and you can check this out by using the reverse DNS lookup feature on the web site. A typical ISP provided PTR record might look something like this… “” (except with a valid IP address). What you want it to look like is this: “”.

Unless you have a good relationship with your Telco/ISP account manager, you’re probably going to have a tough time finding the right person if you’re just calling customer support. So instead of calling them, start by doing a WHOIS lookup on the ISP. What you’re looking for are the hostmaster (usually and abuse addresses (usually They’re typically very responsive and can act as a technical a point-of-contact for escalating and handling things that customer support has no knowledge of.

So what should you tell them in your email? Well, make certain to explain the entire situation - don’t just tell them that you need the PTR changed. Explain everything including the fact that you looked up the public block lists and confirmed that the client isn’t on any, include the fact that you reviewed the firewall’s outbound SMTP configuration, include the fact that you contacted the mail host’s support group, use all of the above points to justify your request for changing the PTR record.

Finally, not all ISP’s will cooperate with this request. My recommendation is to develop a relationship with an account manager at the Telco/ISP that will work this for you, and then use them as your point of contact for everything – from provisioning new service, to making changes to the PTR records. Short of that, your best bet is to follow the above recommendations.

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