Mail Server Error Sent OK but not Received on recipient address

Mail Server Error Sent OK but not Received on recipient address same applied in incoming mail i am not receiving any kind of mail the sender told me your email bounce back.

Reject the request when the hostname in the client HELO (EHLO) command has no DNS A or MX record. The unknown_hostname_reject_code specifies the response code to rejected requests (default: 450