You need to design security for messages exchanged between Northwind Traders employees and its vendors. What should you do？（）

A. Require employees to access e-mail only over connections that use SSL
B. Place Exchange mailbox stores on NTFS volumes. Use Encrypting File System （EFS） to encrypt the stores
C. Use the internal certification authority （CA） to issue certificates to all employees. Require employees to digitally sign outbound e-mail
D. Use the internal certification authority （CA） to issue certificates to all employees and vendors. Instruct employees and vendors to use secure MIME to encrypt all e-mail sent between them