Use the Server app to create a mail exchange (MX) record on your DNS server. If you have an ISP that provides DNS service, contact the ISP so they can enable your MX records.
In the DNS pane, click the Action pop-up menu , then choose Show All Records.
Click Add , then choose Add Mail Exchange Record from the pop-up menu.
Click the Zone pop-up menu, then choose a zone.
The mail exchange record is added to this zone.
Enter the name of your Mail server in the Mail Server field.
This field is the basis for the computer’s MX record.
In the Priority field, specify a mail server precedence number.
Delivering mail servers try to deliver mail to lower numbered mail servers first.
Click Create.