Great investigative journalism (or whatever the correct term is): security research Per Thorsheim does an experiment to see which law-enforcement organizations appear to support TLS encryption for SMTP mail. If you’re wondering why this is relevant, recall that the recent “hack” [I hate using that term since it’s not really a hack as much as it was social engineering] of the FBI-Scotland Yard conference call happened after a member of Anonymous found a conference call invitation in stolen e-mail. That kind of data can have intrinsic value that might make it attractive to an attacker, and using STARTTLS is a conceptually simple way to protect it while in transit.
TLS support in Exchange 2010 has come a long way from the bad old day of Exchange 2003, where you may recall that enabling TLS would cause the SMTP virtual server to refuse to talk to any other SMTP server that wouldn’t accept TLS. Exchange 2007 added support for opportunistic TLS, and Exchange 2010 has it too. Microsoft’s documentation makes clear how to set it up, so I encourage you to stop reading this article and just go do it.