Explore help: click on picture!
When you click Prefs from the main menu you see the preferences panels. This is where you set up your mail accounts and change details about the ways you send and get your mail.
When you select Prefs, it starts at the "Email Accounts" panel which shows all your email accounts.  From here, you can create New accounts, Edit and Delete accounts.
The other panels cover Getting and Sending email, Chatting, Owners, Registration and General preferences.