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: 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.