The SMTP protocol operates at OSI Level:

The Simple Mail Transfer Protocol (SMTP) is an application-layer protocol used for sending electronic mail. In the seven-layer Open Systems Interconnection (OSI) model, the Application Layer is Layer 7. This layer is responsible for providing network services directly to end-user applications. SMTP defines the procedures for mail transfer between mail servers, which is a high-level, application-specific function. Therefore, it operates at Layer 7, interfacing with mail user agents and handling the end-to-end mail delivery process.

A. 23: This is the well-known port number for the Telnet protocol, which also operates at Layer 7, but it is not an OSI layer number.

B. 25: This is the well-known port number assigned to the SMTP protocol for server-to-server communication, not its corresponding OSI layer.

D. 3: This is the Network Layer of the OSI model, which is responsible for logical addressing and routing of packets (e.g., the Internet Protocol - IP).

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1. Kurose

J. F.

& Ross

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

"Application Layer

" Section 2.4

"Electronic Mail in the Internet

" SMTP is explicitly defined and discussed as a primary application-layer protocol.

2. Postel

J. (1982). RFC 821: Simple Mail Transfer Protocol. IETF. The abstract and introduction describe SMTP as a protocol for mail transfer between hosts

a function characteristic of the Application Layer (Layer 7). The protocol's commands (e.g.

HELO

MAIL

RCPT) are application-level interactions.

3. Braden

R. (Ed.). (1989). RFC 1123: Requirements for Internet Hosts -- Application and Support. IETF. Section 5

"Application Layer -- MAIL

" is dedicated entirely to email protocols

stating

"This section discusses the Simple Mail Transfer Protocol (SMTP)..." (Section 5.1

p. 69)

firmly placing it within the Application Layer.