Seeing sensitive e-mail on paper can provide a fresh look at what you're saying. As you edit your note, be sensitive to the words you use. Don't say it by not really saying it. Sarcasm and verbal nuances don't translate well online. For example, it's very difficult to be sarcastic without nonverbal cues, like the tone of voice.
People can miss nuances, especially emotional nuances.
When you're writing, give e-mail your undivided attention. E-mail makes mistakes more convenient. It's easy to accidentally select the wrong distribution list. Or to accidentally forward your sweetheart's love letter to everyone. Or to mistakenly forward your co-worker's joke about the boss to the boss.