Email Encryption Service

I am looking for an inexpensive but efficient service or product that can be used to easily encrypt emails containing confidential information such as Protected Health Information. An Outlook plug-in would be ideal. I have tried ZixMail and Citrix ShareFile and have looked at a product called R-Post – all of which are monthly/yearly subscriptions billed on a per user basis. I was wondering if anyone out there could recommend a product/service? Thank you in advance!