mediumMultiple Choice
220-1201 Practice Question: That they can send emails but cannot receive any
A user reports that they can send emails but cannot receive any. The email client is configured with SMTP for outgoing and POP3 for incoming. The server is reachable. Which port is most likely blocked on the firewall for the incoming mail protocol?
⚠ Common exam trap
CompTIA often tests the distinction between SMTP (port 25) and POP3 (port 110) by describing a scenario where only one direction of email flow is broken, leading candidates to mistakenly focus on the outgoing port.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Port 110
The user can send emails (SMTP works) but cannot receive them, and the incoming protocol is POP3. POP3 uses port 110 by default. Since the server is reachable, a firewall blocking port 110 would prevent the email client from retrieving incoming messages, while SMTP on port 25 remains unaffected.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Port 25
Why it's wrong here
Port 25 is SMTP, used for sending and server-to-server relay, whereas POP3 retrieval listens on port 110. Blocking 25 would prevent outgoing mail, contradicting the symptom of successful sending. It would be correct if the question asked which port a blocked outbound SMTP relay uses.
- ✓
Port 110
Why this is correct
Port 110 carries POP3, the incoming protocol named in the stem, so blocking it prevents retrieval while SMTP on port 25 still sends. The server being reachable rules out connectivity faults, isolating the firewall rule as the cause. Port 995 would apply only to POP3S, which the client is not configured to use.
- ✗
Port 443
Why it's wrong here
Port 443 carries HTTPS traffic, not POP3, which listens on port 110 (or 995 for POP3S). Blocking 443 would break web and API access rather than inbound mail retrieval. It would be the right answer if the client used a webmail interface or Exchange Web Services over HTTPS.
- ✗
Port 53
Why it's wrong here
Port 53 carries DNS name resolution, not POP3 retrieval, so blocking it would break hostname lookups rather than inbound mail. It is tempting because DNS failures do disrupt email delivery indirectly, and port 53 would be the correct answer if the fault were name resolution instead of the POP3 service itself.
Go deeper
Related to this question
Learn chapter
Mobile Email Configuration: POP3, IMAP, Exchange
Key term
CAN
A CAN (Controller Area Network) is a robust vehicle bus standard designed to allow microcontrollers and devices to communicate with each other without a host computer.
Key term
SMTP
SMTP (Simple Mail Transfer Protocol) is the standard internet protocol used to send emails from a client to a server or between email servers.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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