Following from the previous rule, if email is actually the property of the employer, then it makes sense to actually track who is sending emails to whom.
Using the Microsoft Exchange Web Storage System, you should track the number of emails sent internally (i.e. to a colleague) and emails sent to clients in your corporate database.
Figure: Monitor Sent Items
One option is to use a Utility for Exchange Reports called SSW Exchange Reporter.
Note that although it is acceptable for seniors to check juniors' email, it is not acceptable for juniors to check seniors' email.