I am VERY careful with any emails I send. As too many stories attest to, an email can end up in some of the strangest places either through carelessness, hitting the Replay All instead of Reply, or even someone forwarding your email to someone else.
Even in writing emails intended to stay internal to the company that may contain criticism of a fellow employee or a customer, I attempt to keep the language professional and courteous.
Also, I am in a position where I have had the need to review the emails of employees who have been let go. Needless to say, there are often emails in the mailbox that could potentially be embarrassing for both the former employee and for senders / recipients who are still current employees.
In short, my philosophy is that EVERY email is “on the record”. If I need to say something that I wouldn’t want published everywhere, then say it in person, but don’t put it in writing.