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:
In the view that opens, choose the "Email account" option under "Send and receive emails with my:". Then click on "Connect" and configure your email 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.