Search the Help system of your mail server for the terms "SMTP connector," "SMTP relay," or "relay host." Look for a place where you can specify the SMTP relay host or connector for outbound messages. This is where you enter the customer-specific DNS record from the Service IP addresses page in the cloud portal, custXXXX-s.out.mailcontrol.com. If you cannot locate a place for this, contact your mail server vendor for assistance.