Between email servers on the Internet, which protocol is used to transfer messages from one server to another?

Introduction / Context:
Internet email delivery relies on a standard server-to-server transport protocol. Understanding the division of responsibilities among SMTP, POP3, and IMAP clarifies how a message moves from the sender’s domain to the recipient’s server and then to end users.

Given Data / Assumptions:

• We focus on inter-server transfer over the public Internet.
• End-user retrieval protocols are out of scope for this hop.
• Standard ports and roles apply.

Concept / Approach:
SMTP (Simple Mail Transfer Protocol) is the application-layer protocol used for message submission and relay between mail transfer agents (MTAs). POP3 and IMAP are for client retrieval from a mailbox. FTP transfers files, not email messages; SNMP is for network management.

Step-by-Step Solution:
Identify the hop in question: server A → server B.Match to protocol: SMTP handles relay using DNS MX records and TCP (commonly port 25).Exclude client retrieval protocols (POP3/IMAP) and unrelated ones (FTP/SNMP).

Verification / Alternative check:
Inspect message headers (Received: lines) to see SMTP handoffs between MTAs during delivery.

Why Other Options Are Wrong:
POP3 / IMAP: Used after delivery for users to access their mailboxes.

FTP: General file transfer, not email relay.

SNMP: Monitoring/management, not message transport.

Common Pitfalls:
Confusing message submission (SMTP) with retrieval (POP3/IMAP). Both involve email but different directions and roles.

Final Answer:
SMTP