Tapping the principles of utility, curiosity, and specificity, if I were to send you an e-mail pitch about the preceding five paragraphs, I might use this subject line if I suspected your inbox was jammed: 3 simple but proven ways to get your e-mail opened. But if I thought you had a lighter e-mail load, and you already knew me well, I might use: Some weird things I just learned about e-mail.
