If you are sending emails containing medical patient information, personal information such as social security numbers, and private company information; then you need to make sure that this information if kept secure.
I recommend that you look at ZixMail. They have a network of over 20 millions members.
Zix Encrypted Email has a solution whether you need just a few users or have hundreds of users.
Organizations who need this are: Health Care Providers, Law Firms, CPA firms, and anyone who sends private information.
For details see: ZixMail Datasheet
Prevent Data Security Breaches with Email Encryption
Email Encyption Article on Minnesota Independent Insurance Agents & Brokers Website