In Internet mail protocols, what is the default TCP port number used by POP3 (Post Office Protocol version 3) for retrieving emails from a server?

Introduction:
Mail clients commonly use standardized Transmission Control Protocol (TCP) ports to connect to servers. POP3 (Post Office Protocol v3) is one such protocol used to download messages from a mailbox. Knowing the default port numbers helps with firewall rules, troubleshooting, and exam readiness.

Given Data / Assumptions:

• We are concerned with the default port for POP3 without TLS implicit wrapping.
• Other mail protocols (IMAP, SMTP) use different well-known ports.
• Security variants may use alternate ports.

Concept / Approach:
POP3 operates by default on TCP port 110. If using implicit TLS (“POP3S”), servers typically listen on TCP port 995. IMAP uses port 143 (and IMAPS 993), and SMTP servers commonly accept message submissions on ports 25, 465 (implicit TLS), or 587 (submission). Recognizing these distinctions prevents misconfiguration during client setup.

Step-by-Step Solution:
1) Identify the protocol: POP3 for message retrieval.2) Recall well-known port mapping: POP3 → 110.3) Differentiate from IMAP (143) and HTTP (80), which are often confused by beginners.4) Provide context about secure variants (995) without changing the default answer.

Verification / Alternative check:
Mail client documentation and IANA port assignments list POP3 with default TCP port 110, confirming the mapping.

Why Other Options Are Wrong:

• 90: not a standard mail protocol port.
• 80: HTTP default port.
• 49: historically used by TACACS, not POP3.
• 143: IMAP default port, not POP3.

Common Pitfalls:
Confusing POP3 with IMAP or assuming the secure variant (995) is the “default.” Unless the question explicitly asks for the implicit TLS port, 110 is the correct default.

Final Answer:
110