The appeals were the product of rigorous experimentation by a large team of analysts. "We did extensive A-B testing not just on the subject lines and the amount of money we would ask people for," says Amelia Showalter, director of digital analytics, "but on the messages themselves and even the formatting." The campaign would test multiple drafts and subject lines—often as many as 18 variations—before picking a winner to blast out to tens of millions of subscribers. …

It quickly became clear that a casual tone was usually most effective. "The subject lines that worked best were things you might see in your in-box from other people,” Fallsgraff says. " 'Hey' was probably the best one we had over the duration." Another blockbuster in June simply read, "I will be outspent."

Propublica archived many of the messages and their variants.