Letters never sent.

The ‘draft’ feature is the most important email feature you have.  Harry Truman left 140 letters in his desk on his death.  “The best letters I ever wrote were the ones I never sent.”  He’d dash off an angry, profane diatribe (referred to Nixon as “Squirrel Head”) then put the letter in the drawer to send the next day.  Which he never did.  Never, ever send an email in anger, when it’s late, you’re jet-lagged, or had too much to drink.  Write it, save it as a draft, then look at it the next day and realize how smart you were to wait, and what a fool you’d have been to send it.