If you have System Setup > Mail Relay > SMTP Controls > Unknown Sender Domain enabled a mail will be rejected if the sending email domain does not have an 'A' or 'MX' record i.e. the domain has to be missing both an A and MX record for mail to be rejected.  


You can't get around this with:


- Whitelisting IP Address

- Putting the HELO in the the Allowed HELO Hostname Restrictions


One way to get around to get around it is to create a DNS zone and an A record for the sending on the "local" DNS servers.  The IP address of the A record does not have to match where the message is coming from.  You just need the "A" to make it so that the sending domain is no longer "Unknown".