Many email users are surprised when they find out how insecure standard email really is.  There are two major gaping holes in email security.

First, from the moment you click send or receive, email is transmitted over the public Internet in clear readable text.  With minimal effort and basic technical know-how, anything considered private or sensitive is open to prying eyes with a mission. 

Second, email addresses are subject to easy forgery and spoofing.  It is trivial to pretend to be someone else on the Internet.  You might receive and reply to an email from a bad guy pretending to be someone you trust.  Or, your client might receive and reply to an email from a bad guy pretending to be you!

Are you compliant with your industry standards or concerned about privacy?  Do you have to safeguard personally identifiable information, health information, credit card information, or other sensitive data?