From the Nicci French Blog.
Although email has the word ‘mail’ in it, it’s not like a letter and although it’s sent over a telephone line, it’s not like a telephone call. What it’s more like is the postcard you send from holiday that you send to someone in the office which they then pin up on the wall. You don’t free-associate on a postcard about kneecapping your enemies. You don’t spread vicious gossip about close friends on a postcard. There are more appropriate channels for such things.
My own rule about my own emails is this: if there is anything that embarrass me if it were printed out, put on a giant poster and pasted up in Piccadilly Cirucs, then I cut it out.