The forward lookup (A) of the hostname hostname did not match the reverse lookup (PTR) for the IP Address.
Example of a correctly matching pair of records:
(A) lookup for smtp.mxtoolbox.com resolves to 188.8.131.52
(PTR) lookup for 184.108.40.206 reverses to smtp.mxtoolbox.com
Some receiving mail servers may use this as an indication of a possible spam source in a scoring system. Most will not reject incoming mail solely on this basis. We recommend that you contact your ISP and ask them to setup a reverse record (PTR) that matches the hostname of your mail server.