A keyboard is required. Shortcuts work only with a hardware or Bluetooth keyboard attached to the iPad.
The set is contextual. Different screens expose different shortcuts. The email list, the composer, drafts, search, and the sidebar each have their own keys.
Composing. Cmd+Return sends, Shift+Cmd+F, Shift+Cmd+C, and Shift+Cmd+B set the From, Cc, and Bcc fields, and Tab moves to the next field.
Email list. Cmd+R replies, Shift+Cmd+R replies all, Shift+Cmd+F forwards, Cmd+Backspace archives, and Backspace deletes the selected message.
Navigation. Arrow keys move between messages, Return opens or selects, page up and page down scroll, and Cmd with the left or right arrow opens or closes the sidebar.