I have and old Nagios server on CentOS 5.11 and want to migrate to a newer CentOS 7 machine. I copied the config, but the e-mail sending isn't working, got the following error:

Sep 11 15:23:38 mon postfix/smtp[13246]: 6B737E0F92: to=, relay=mx1.example.net[xxx.xxx.xxx.xxx]:25, delay=176789, delays=176784/0/5.1/0.04, dsn=4.7.1, status=deferred (host mx1.example.net[xxx.xxx.xxx.xxx] said: 450 4.7.1 Client host rejected: cannot find your reverse hostname, [] (in reply to RCPT TO command))

What can I do? I searched a lot on Google, but didn't find any solution. I don't have any DNS settings on the old and the new server too.

  • If you need specific information on how to configure reverse DNS, please provide relevant information like address space used, private or public, assigned by whom, whether the mail relay server which you camouflaged as mx1.example.net is or isn't under your control, and whether NAT is involved. – Tilman Schmidt Sep 11 '15 at 14:45
  • It would also help if you posted the original, unmangled log message. Your mangling destroyed lots of important information. – Tilman Schmidt Sep 11 '15 at 14:53

You need to configure reverse DNS so that a lookup of your new Nagios server's IP address from the mail relay server will yield the correct FQDN of your new Nagios server, or at the very least an FQDN which will resolve back to the same IP address.

  • But as I mentioned earlier, I don't have a Domain name for both servers. If I use a gmail address, it works well. – Roberto Sep 11 '15 at 14:45
  • 1
    You may want to clarify your question then. You wrote that you don't have DNS settings on the servers. Now you say you don't have domain names for them. That's two different things. Also please explain what you mean by "if I use a gmail address". – Tilman Schmidt Sep 11 '15 at 14:51

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