You can find all email settings by clicking on the 'Settings' menu and then selecting the 'General' submenu.
Q: I have an iCloud account. How can I have Billit send emails from my own email address via the iCloud servers?
A: You will need the server data of your iCloud account. As an extra security measure you will need to create an app-specific password. You can read on this Apple help page how you can do that. Use the server data and this new password to set up your SMTP server in Billit.