Database Mail Exception 5.7.1

The mail could not be sent to the recipients because of the mail server failure. (Sending Mail using Account 1 (2015-08-11T17:20:13). Exception Message: Cannot send mails to mail server. (Mailbox unavailable. The server response was: 5.7.1 Unable to relay for [email protected]). ) <p style="text-align:justify;">Although it looks like that there is an issue with the account that we're trying to send the message to, this is not actually the case: The Exchange Server requires a new Receiver Connector to be created so that the Exchange server can relay messages from the server.