You need to design access to e-mail by Internet users. What should you do？（）

• A、Configure front-end servers to use HTTP to communicate with back-end servers
• B、Configure the internal firewall to allow IPSec traffic between front-end and back-end Exchange servers
• C、Require all users to encrypt all outbound e-mail messages
• D、Issue digital certificates to all remote users. Require the certificates to be used when authenticating to Outlook Web Access