People are corruptible no matter what their job title is, so there's no reason that extra security checks shouldn't be put in place. If sensitive information is being stored on an email server, it doesn't matter if it's secure or not, because a corrupt cabinet member have an easy-access way to communicate state or federal secrets to someone that has no right knowing such things. With a federally monitored email server, that's one less way of communication for any corrupt business.