Administrators often forget that management of emails isn't simply a question of who can send or receive them, or the content. When email arrives it sits in the inbox, but who has permission to access that inbox, or other folders for that matter?
With high turnovers of employees becoming increasingly common, the stressed administrator may well lose track of individual permissions. For example, many companies usually have a set of freelance or contract email accounts that can be assigned to temporary staff. The permissions required by one contractor may be far more extensive than those required by their successor, but they could very well inherit those permissions when they arrive. And human nature being what it is, inquiring minds will almost certainly take advantage of this, whether innocently or maliciously.