1. Login to the Members Control Panel at https://controlpanel.netfirms.com
2. Click Domains
3. Click Domain Manager
   
4. For the domain you wish to configure, click the corresponding plus sign
5. Click the DNS tab
   
6. Click MX Record
7. Enter your custom MX Records
8. Click Update
   

NOTE: On any Netfirms Premium Hosting Plan you may add up to 3 MX records. Please note that changes to your MX Records will require up to 72 hours to fully propagate throughout the Internet.  

On a Domain Parked Plan you may configure 1 MX Record