Client host rejected: cannot find your reverse hostname

Suddenly I can send any mail anymore. When I look in the logfile I see my mails being rejected with this error message “Client host rejected: cannot find your reverse hostname”. As far as I can’t remember I didn’t change a thing on the settings in the period when the change happened.

It is an Ubuntu site with Postfix. All mail commands are sent from PHP.

What could this be?

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Sometimes I get the alternative message (from a gmail address) “This message does not have authentication information or fails to pass 550-5.7.1 authentication checks. To best protect our users from spam, the 550-5.7.1 message has been blocked. Please visit 550-5.7.1 for more 550 5.7.1 information.”

Note that this worked before. And I have another server where it still works.