I'm surprised it hasn't happened before. When I was in the air force, it was common practice to send out embarrassing emails if someone left their computer unlocked. They'd send them to the whole squadron or wing if they could.
Some of the emails were embarrassing and very funny.
I've pointed out that this technically breaches company policy regarding use of someone else's user account/identity, but it falls on deaf ears.
Agreed. But at this point, only one person has breached policy and it's a forgetfulness/oversight kind of breach.
If someone actively takes control of the machine to send an email, that's a much more serious offence.