Within the TCP/IP suite, which protocol is used to transfer electronic mail messages between mail servers (host to host)?

Introduction / Context:
Email delivery on the Internet relies on a standardized application-layer protocol for relaying messages between servers. This question assesses your knowledge of which TCP/IP protocol performs server-to-server mail transfer, not client retrieval.

Given Data / Assumptions:

• We focus on the transport of messages between hosts (Mail Transfer Agent to Mail Transfer Agent).
• Client retrieval protocols like POP3 or IMAP are out of scope here.
• Standard TCP/IP protocol stack nomenclature is assumed.

Concept / Approach:

Simple Mail Transfer Protocol (SMTP) is the de facto Internet standard for pushing emails between servers. Mail servers accept, relay, and deliver messages via SMTP over TCP, commonly on ports 25, 465 (with TLS), or 587 (submission). Other listed protocols serve different purposes.

Step-by-Step Solution:

Verification / Alternative check:

Examine a real email header: the Received lines show SMTP hops between servers, confirming SMTP usage for relay.

Why Other Options Are Wrong:

FTP: File transfer, not email relay.

SNMP: Network monitoring and management, unrelated to mail transport.

RPC: Programming abstraction for calling procedures on remote systems, not a mail protocol.

None of the above: Incorrect because SMTP is the standard.

Common Pitfalls:

Confusing SMTP with POP3/IMAP, which are for retrieving emails from a mailbox to a client, not inter-server transport.

Final Answer:

SMTP