The Obama campaign also found that even though people might be annoyed by the large number of email pitches popping up in their Inbox, that did not trigger an automatic unsubscribe. 

Still, a study by email intelligence firm Return Path in October found that less than half those emails are read on a regular basis. Obama's team told Bloomberg Businessweek it was nearly impossible to predict the email content or formatting that would prompt action, which is why the team was continually testing different ideas.

“We were so bad at predicting what would win that it only reinforced the need to constantly keep testing,” Amelia Showalter, Obama's director of digital analytics, said. “Every time something really ugly won, it would shock me: giant-size fonts for links, plain-text links vs. pretty ‘Donate’ buttons. Eventually we got to thinking, ‘How could we make things even less attractive?’ That’s how we arrived at the ugly yellow highlighting on the sections we wanted to draw people’s eye to.”

The team also found that mild profanity caught people's attention, such as “Hell yeah, I like Obamacare." And the subject line “I will be outspent,” warning of Republican super-PACs, raised more than $2.6 million alone.