i receive a message of 5.1.2 - Bad destination host "DNS Hard Error looking up mydomain.com' (MX): -- error message received by sender.
This happen to one of the user want to send email you our mail server.
we find no record of email received from the sender from our sid. meaning message has not reached my server and i have checked the spam blacklisted and found that there is a block from SORBS-SPAM.
Due to that problem i have added sender email to the whitelists of my spam mail firewall and mail server.
Above solution doesnt solve the problem the email send by sender still bounce.