Email is transmitted across the Internet using the Simple Mail Transport Protocol. SMTP does not encrypt email, leaving the information in the…

The STARTTLS command is an extension to the Simple Mail Transfer Protocol (SMTP) that allows an unencrypted connection to be upgraded to a secure connection using Transport Layer Security (TLS). When a client connects to an SMTP server, it can issue the STARTTLS command to signal its desire to begin a TLS handshake. If the server supports this extension, it responds positively, and the two parties negotiate a secure, encrypted channel before any sensitive information, such as login credentials or email content, is transmitted. This process is defined in RFC 3207.

A. OPPORTUNISTICTLS: This is a configuration policy where a server attempts to use TLS but will proceed with an unencrypted connection if TLS fails; it is not an SMTP command.

B. UPGRADETLS: This is a descriptive but syntactically incorrect and non-existent command within the SMTP protocol specifications.

C. FORCETLS: This describes a policy that mandates the use of TLS for a connection, refusing to proceed if a secure connection cannot be established. It is not a command.

1. Internet Engineering Task Force (IETF). (2002). RFC 3207: SMTP Service Extension for Secure SMTP over Transport Layer Security. Section 4

"The STARTTLS Command

" p. 3. This document formally defines the STARTTLS command

its syntax

and its function in upgrading a plaintext SMTP connection to a TLS-encrypted one.

2. Kurose

J. F.

& Ross

K. W. (2021). Computer Networking: A Top-Down Approach (8th ed.). Pearson. In Chapter 2

Section 2.4.3

"Mail Access Protocols

" the text discusses SMTP and its security extensions

including the use of STARTTLS to encrypt email sessions.

3. MIT OpenCourseWare. (2017). 6.857 Computer and Network Security

Fall 2017. Lecture 15: Network Security II. The lecture notes discuss email security mechanisms

explicitly mentioning STARTTLS as the command used to initiate TLS encryption within an SMTP session.