You can check the MX record or the domain in question whether it really points to your server.
Then you can check what happens when you send a mail. Do you get it back with an error message? What error message?
Then you can check if it is really true that the server does not receive mail from the outside by checking /var/log/maillog (mail.log) to see what is going on at the mail server when mails are coming in.
Further action depends on the results.