use MAIL_LOCATION for mx record

src/config.js

@@ -28,6 +28,7 @@ exports = module.exports = {
     internalAdminOrigin: internalAdminOrigin,
     sysadminOrigin: sysadminOrigin, // caas routes
     adminFqdn: adminFqdn,
+    mailFqdn: mailFqdn,
     appFqdn: appFqdn,
     zoneName: zoneName,
     adminEmail: adminEmail,
@@ -167,6 +168,10 @@ function adminFqdn() {
     return appFqdn(constants.ADMIN_LOCATION);
 }
 
+function mailFqdn() {
+    return appFqdn(constants.MAIL_LOCATION);
+}
+
 function adminOrigin() {
     return 'https://' + appFqdn(constants.ADMIN_LOCATION);
 }