What works as an E-mail message transfer agent?

A.
SMTP

B.
SNMP

C.
S-RPC

D.
S/MIME

Explanation:
In e-mail clients SMTP works as a message transfer agent and moves the message from the user’s computer
to the mail server when the user sends the e-mail message.
Incorrect Answers:
B: SNMP is used for monitoring the network, not for sending email messages.
C: S-RPC is used for remote procedure not calls, and not for sending email messages.
D: S/MIME is a standard for email encryption. It is not used to send email messages.

Harris, Shon, All In One CISSP Exam Guide, 6th Edition, McGraw-Hill, New York, 2013, p. 599