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 220.127.116.11
(PTR) lookup for 18.104.22.168 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.
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