If you invited 100 friends to a party at your house, would you judge the success of the party based solely on the number of people who RSVPed? Of course not. You'd also take into consideration how the party itself went — how many people stayed longer than a few minutes, whether they seemed to have a good time and even how much food and wine was left over. So why would you judge the success of your e-mail campaign based solely on the campaign metrics?