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.
Are you managing your email delivery?
Email delivery is more than running an email server, or regularly checking for blacklists. To really ensure you customers get your email, you need to setup SPF, DKIM and DMARC. If you aren't manage your DMARC configuration and responses from email providers, you don't know if your customers are getting your email.
MxToolbox is YOUR expert on email deliverability. MxDelivery Center can help you setup and analyze your DMARC, DKIM and SPF to give you the insight you need to make email configuration changes and get your emails to your customer's.