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