How can you accomplish this?

You want to secure your SMTP transmissions from sniffing attacks. How can you accomplish this?

A.
Forbid relaying.

B.
Enforce masquerading.

C.
Use an SSL certificate.

D.
Use strict bounds checking on arrays.
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Explanation:
The best defense against sniffing, also know as man in the middle attacks, is to use strong encryption. SSL (Secure Sockets Layer) can be used to implement strong security for SMTP transmissions. SSL uses certificates to implement security.
Incorrect Answers:
A: To prohibit relaying would enable you to control the receipt and delivery of mail. Here however, we want to prevent anyone sniffing the SMTP traffic between the sender and receiver and relaying would not be of any use.
B: Masquerading is a technique is a Network Translation (NAT) scheme that is used to let many clients use the same IP address. It would not help against sniffing attacks however.
D: Strict bounds checking on arrays cannot be applied to SMTP traffic.