Roger Mathus of Death by Email quotes Elizabeth Charnock, CEO of Cataphora on 10 things you should probably never write in an e-mail:

1. “I could get into trouble for telling you this, but…”
2. “Delete this email immediately.”
3. “I really shouldn’t put this in writing.”
4. “Don’t tell So-and-So.” Or, “Don’t send this to So-and-So.”
5. “She/He/They will never find out.”
6. “We’re going to do this differently than normal.”
7. “I don’t think I am supposed to know this, but…”
8. “I don’t want to discuss this in e-mail. Please give me a call.”
9. “Don’t ask. You don’t want to know.”
10. “Is this actually legal?”

He suggests that if you find yourself writing this, you probably shouldn’t do it to begin with.

h/t to Anne Reed, who adds “do not forward,” Walter Olson, and Pete Warden.

Updates

The ABA Journal picks up on this too.