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I’m feeling lost in DNS; please help me get back on track.

  1. I’ve got a domain. Let’s call it And I can add DNS entries for subdomains of type A and CNAME.
  2. I have another server which I want to be available as And I’ve got a mail server there.

When I send emails to certain servers from they get rejected as spam, based on the absence of a PTR record. So, the first question is, What can I do to fix this?

Right now I only have an A record at the DNS of the registrar of I can add all kind of DNS entries for using name servers of another provider. Right now there no such entries.

And the second question is, Can I pull this off without downtime?

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up vote 7 down vote accepted

The best bet to getting a working mail setup in DNS is to follow a checklist. Mine usually looks something like so:

  1. Make sure standard DNS points at
  2. Make sure reverse DNS points at
  3. Make sure the mail server issues "HELO" commands as
  4. Make sure has an MX record pointing to

These can be checked as follows (assuming linux command line):


$ dig +short


$ dig +short -x


$ telnet 25
> 250 Hello [], pleased to meet you


dig +short mx

The above posters are all correct in that your ISP is normally responsible for #2. Sometimes you need to have them change this, sometimes they will delegate the responsibility to you (which means you have to run a DNS server to serve these requests).

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No downtime needed, but you do need to control your IP address space. If you don't, your ISP may be willing to create a PTR record for's IP address.

The PTR record is just a way of (pseudo-definitively, if you will) reverse resolving an IP address to a host

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Did your ISP delegate reverse DNS resolution to you? If not, ask them to add a PTR entry for If yes, you'll need to add a zone with a PTR record for

There should be no downtime at all.

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If someone's rejecting your mail because you have no PTR record for the IP address of the machine sending the e-mail, or the PTR doesn't match the HELO, all you have to do is add/fix that PTR record -- which is just a matter of contacting whoever manages RDNS for the range you're in (I'm going to hope you don't have a /24 or better of your own). There will be no downtime in this.

On the other hand, if someone's rejecting your mail because the sender domain doesn't match the PTR record, please beat them with a stack of printed RFCs.

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