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 Acce
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