How to View an Email Message's Source in Sparrow

"What's a MacGuffin?" asks one of two men in a train's compartment.
"Well, it's an apparatus for trapping lions in the Scottish Highlands," explains the other, pointing at his bulging bag on the luggage rack.
The source for this whimsy story (which does go on) is Alfred Hitchcock, of course, pressed to explain the origin of "MacGuffin", a plot device in films that grapples the actors' attention but may turn out to be nothing at all. The "Mac" in "MacGuffin" possibly stems from Angus MacPhail, the screenwriter and Hitchcock friend who may have coined the term; the "Guffin" could come from "guff" as "nonsense" or "guffaw" perhaps; finally, there's the lion-bodied "griffin" as a possible source for the MacGuffin's sound.
So, the source of the MacGuffin is unclear and not so easy to uncover — quite unlike an email's source in Sparrow, which is easy to get and clear to see:
›› Want to find out what makes an email display the way it does, whence it came and other usually hidden information? Here's how to see an email's full source code in Sparrow.