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. Add an MX record with priority 10 and value mx.yourdomain.com.cust.b.hostedemail.com (replace 'yourdomain.com' with your own domain), and also add a TXT record with value v=spf1 include:_spf.hostedemail.com ~all.
Step 3. Save and apply your changes.
Note: Make sure there are no additional MX records, and also make sure there are no other TXT records with values that begin with v=spf1.