Step 1: Open Mailbutler
Step 2: Attach the Sidebar
While Apple Mail is in full-screen mode, click the Attach button in the Mailbutler Sidebar.
Step 3: Move the Sidebar next to Apple Mail
1. Click and hold the Mailbutler Sidebar by its title bar.
2. While holding the Sidebar, press F3 to open Mission Control.
3. Drag the Mailbutler Sidebar next to the Apple Mail window.
4. The Mailbutler Sidebar will now appear side by side with Apple Mail in full-screen mode.