When the Mac Power Users recorded its recent Mail show, I lamented about the trouble with full screen Mail and writing new messages. Specifically, the compose window is modal and takes over the screen so if you want to refer to a prior e-mail you have exit full screen or cancel the composition.
Reader Sunit figured out a work around. While composing an e-mail in full screen, pressing ⌘⌥N creates a new mail view window. You can search, block, and copy to your hearts delight and then return to the screen with your e-mail composition.