NationBuilder staff will need to add the MX records and finish the custom domain if your email provider is not listed in the drop down menu on the custom domains page. Contact NationBuilder staff before changing the nameservers so we can evaluate which records to use with the domain. Please provide the name of your domain in the email. Once the email has been received we'll begin the custom domain process let you know when the nameservers can be changed in your domain registrar. 

Using the CNAME method for your custom domain will not require add any MX records by NationBuilder staff. You will, however, need to add the following records on this page to your domain registrar.