Using Mailboxes With Your Own DNS
To use a Mailbox with your own DNS follow these steps.
Step 1. Login to your domain registrar account. (Examples: Godaddy, Google, Register.com, etc.)
Step 2. Update the MX records using your domain name in the following format. mx.yourdomain.com.cust.b.hostedmail.com
Step 3. Save and apply your changes.