To use your custom domain in Fleep, you need to connect your email account with Fleep. Just click on your profile in the bottom left corner of the user interface, and choose Email settings from the menu that appears:
If you use Gmail or Google Apps, you can choose to send and receive emails with your Gmail account with just the click of a button.
If you use any other email provider, select the option to send and receive emails with your email account. Then click on "Connect" and configure the setup in the popup window that appears.
Once successfully connected, you will always be able to see and modify your setup from the Email settings view: