Your network contains two Exchange Server 2010 Edge Transport servers and five Exchange Server2010 Hub Transport servers. All e-mail sent from your organization to the Internet is transferred by the Edge Transport servers. You need to recommend a security solution for the organization to meet the following requirements： .Ensure that users can send encrypted messages to any other organization on the Internet .Ensure that all Exchange-related communication between Hub Transport servers and Edge Transport servers is encrypted What should you recommend？（）

A. Deploy IPsec.
B. Deploy SMTP over SS
C. Implement Domain Security.
D. Implement Secure/Multipurpose Internet Mail Extensions （S/MIME）.