In 1921, a young copywriter named John Caples sat down and wrote "They Laughed When I Sat Down at the Piano" for the U.S. School of Music.  The headline was a huge success. And Caples knew precisely how successful it was because the advertisement he wrote contained a coupon - marketing's first scientific measurement tool. 
Caples was an early champion of testing and his book, "Tested Advertising Methods," added an element of science to selling. The book is still in print and continues to draw raves from Amazon reviewers.
The power to measure campaign results has exploded in the past few years. And as the Internet continues to level the playing fields of marketing, testing e-mail campaigns is a bigger deal than ever.  
"If you can't measure it, you can't manage it," management guru Peter Drucker said. We'd only add beware of spending time and money measuring parameters which won't be actionable.
So what should e-mail marketers test?