Microsoft has published some information about their internal need for trustworthy messaging, and how they solved it.
They needed to use email for transmitting business-sensitive data but was concerned about security and data privacy. Microsoft's internal IT group deployed a solution they called trustworthy messaging, offering both senders and recipients three levels of protection for their data. These solutions were based on applying Secure Multipurpose Internet Mail Extensions (S/MIME) digital signatures, S/MIME encryption, and/or rights management policies.
Anyways, if you are interested Microsoft has released a small slidedeck (not that useful) and a Word document (much better than the slidedeck) that gives a glimpse at their thought process on the topic. [Dana Epp's ramblings at the Sanctuary]