You need to design a method to modify the current e-mail ordering system. Which will be used until Fourth Coffee deploys the Web-based ordering application. Your solution must address business concerns and improve security. What should you do？（）

• A、Configure the mail server to disallow SMTP relaying
• B、Instruct customers to obtain digital certificates from a trusted commercial authority （CA）， and digitally sign all order e-mail messages. Reject unsigned order e-mail messages
• C、Provide customers with a public encryption key， and instruct them to encrypt all order e-mail messages. Reject unencrypted e-mail messages
• D、Implement an e-mail filtering solution， and add customer e-mail addresses to the list of allowed addresses. Reject e-mail messages from other addresses