Client host rejected

I get the error message “Client host rejected: cannot find your reverse hostname” – what does it mean?

Due to recurring massive spam abuse from exploited hosts, Cleanmail.ch only accepts mail from servers with correctly defined, non generic rDNS/PTR entries.

This sender server’s IP address is missing rDNS/PTR record. We recommend setting the PTR to a non generic hostname in a domain managed by the server operator.