In this article you will learn how to add the MX records required to enable e-mail through your custom domain after you have followed our guide at Setting Up a Custom Domain Using CloudFlare.
The basic principle involved is to:
1.) Create an A record for "mail", and then point that record to the mail server's IP address (which you can get from your host).
2.) Create an MX record and point that to the mail server.
Please see the video below for more details.