Secure email
Securing the mail server isn't that hard if you are willing to make an effort - it's securing the users that's the real problem - serious security tends to make life harder for your users and, while they will usually not devote much effort to following recommended security procedures, they are often wiling to devote quite a bit of effort to work around them.
And the NSA adds in a footnote, whatever you do, don't use an iPhone - even the FBI can break into those.
Biting the hand that feeds IT
