smtp;450 4.7.1 Client host rejected: cannot find your hostname

When sending an email to a user, I get the following NDR

You do not have permission to send to this recipient.  For assistance, contact your system administrator.
            <mailserver.mydomainname.com #4.7.1 smtp;450 4.7.1 Client host rejected: cannot find your hostname, [My Mail Public IP]>
Ali_AamerAsked:
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Ali_AamerConnect With a Mentor Author Commented:
Added Reverse DNS entry to the public IP of my mail server by contacting the ISP.   Solved the problem.
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