I need a list of legitimate companies that offer encrypted email.?

My boss has asked me to research encrypted email. If someone with experience in setting this up for their company or someone that knows quite a bit on the subject please advise me of the steps I need to take in order to accomplish this.
Update: We're talking about the security of thousands of people here, so please don't just give me a link. I'm concerned about the legitimacy of any company that I just google...can someone with experience with setting this up for their company please send me in the right direction?
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