I have been tasked with finding this same solution. We have been running a proof of concept trial on SafeSend (https://www.safesendsoftware.com). It is essentially an Outlook plug-in. For all external e-mail it prompts the sender to verify the intended recipients. There is also an option to turn on to verify any attachments as well. The price quoted is a little more than I would want to pay for a plug-in, but considering we had 3 instances last year of sensitive e-mail sent to incorrect recipients we think it's worth it.
That looks interesting. For where I work. One email could wreck a lawsuit worth millions upon millions of dollars. So when compared to that most of the time cost isn't a factor.