SMTP can best be described as:

A.
a host-to-host email protocol.

B.
an email retrieval protocol.

C.
a web-based e-mail reading protocol.

D.
a standard defining the format of e-mail messages.

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: SMTP is used only for sending, not retrieving, email messages.
C: SMTP is used only for sending, not reading, email messages.
D: SMTP is not a format of email messages. It is a protocol for sending email messages.

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