System Administration & Network Administration
domain-name-system email ptr-record
Updated Sat, 20 Aug 2022 13:59:07 GMT

Does a PTR record prove anything about the sender's email domain?


My outgoing mail server has problems reaching some recipients. This happened after we changed ISP for our dedicated IP address. I think it might be due to PTR records, but I cannot be sure.

My assigned IP address is x.y.z.112/29. When I do an nslookup on x.y.z.114 (WAN-facing public IP address), it gives 114.x-y-z.myisp.com. Am I right to say that there is indeed a PTR record set for my IP address, only that it does not match my MX mail.mycompany.com. (x.y.z.115)?**

I also learned that the extent to which mail servers check PTR records varies. Some only check that a reverse DNS lookup (rDNS) hostname exist while some go all the way ensuring that the MX and the rDNS hostname matches. So what should I do? Should I still tell my ISP to set PTR records for mail.mycompany.com?

So now my PTR record resolves to 114.x-y-z.myisp.com whose A resolves to the same IP address as the PTR record. So what does this prove about the sender's email address?

Solution

To answer your new question first: No, PTRs don't tell you anything about the sender's domain. See below for the explanation.

Now back to your original question:

Receiving mail servers check none, one, many or all of the following:

  1. Is the HELO name equal to the hostname (A record)?
  2. Is PTR of the IP equal to the A record of the hostname (hostname ==(PTR)==> IP ==(A)==> hostname)?
  3. Is the IP part of the provided SPF record?
  4. Does the sender's domain have at least one MX record? Which doesn't have to match IP/hostname.

Receiving mail servers that check if the sending server is also the MX server are badly configured and should be eliminated from the Internet.

Edit: The PTR does absolutely prove nothing about the email domain. It is never meant to prove this. There are thousands of domains hosted at Google, Amazon, AOL and others. But none of them match the hostname or PTR of Google, Amazon, AOL and others. They all have the servernames of the providers. And there is nothing bad about that.

The PTR only proves the identity of the server but not the identity of the hosted domains. Point.

2nd Edit: A good example for a working environment would be

  • HELO = mail.example.com
  • hostname = mail.example.com
  • A record of mail.example.com = 172.20.25.25
  • PTR of 172.20.25.25 = mail.example.com
  • Domains hosted on this server/system = example.com, *.example.com, *.example.net, *.example.org, mycompany.invalid and many many more.
  • SPF records of hosted domains (optionally) = v=spf1 a:mail.example.com -all
  • MX record of hosted domains can be anything. E.g. mx1.example.com, mx2.example.com, mailfilter.anti-spam-corp.invalid, mail.example.com, postini.google.invalid, ...


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References