- A
POP3 on port 110
Why wrong: POP3 is for receiving email; blocking it would affect receiving, not sending.
- B
IMAP on port 143
Why wrong: IMAP is for receiving email; blocking it would prevent receiving, not sending.
- C
SMTP on port 25
SMTP on port 25 is used for sending email; blocking it prevents outgoing messages while receiving works.
- D
HTTPS on port 443
Why wrong: HTTPS is for secure web browsing; blocking it would affect internet access, not email sending specifically.
Question 437 of 1,020
SMTP Port Blocked: Email Cannot Send But Can Receive
This 220-1201 practice question tests your understanding of network protocols. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A user reports that their email client can receive messages but cannot send any. The email server settings are correct, and the user can connect to the internet. The firewall is blocking outbound traffic on certain ports. Which protocol and port combination is likely blocked?
Quick Answer
The answer is SMTP on port 25, as this is the protocol and port combination most likely blocked when a user can receive email but cannot send. SMTP handles outgoing mail, while receiving relies on separate protocols like POP3 or IMAP, which use different ports; if the firewall blocks outbound traffic on port 25, the email client fails to send messages even though internet connectivity and server settings are correct. On the CompTIA A+ Core 1 220-1201 exam, this scenario tests your understanding of common firewall restrictions and email protocol roles—a frequent trap is confusing SMTP’s sending function with receiving protocols or assuming port 587 is always the default. Remember the memory tip: “SMTP sends, so port 25 is the sender’s gate; if it’s blocked, your outbox waits.”
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
✓
SMTP on port 25
SMTP (Simple Mail Transfer Protocol) is used for sending outgoing email messages. By default, SMTP uses port 25. If the firewall blocks outbound traffic on port 25, the email client can still receive messages (using POP3 or IMAP) but will be unable to send any, perfectly matching the user's symptoms.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
POP3 on port 110
Why it's wrong here
POP3 is for receiving email; blocking it would affect receiving, not sending.
- ✗
IMAP on port 143
Why it's wrong here
IMAP is for receiving email; blocking it would prevent receiving, not sending.
- ✓
SMTP on port 25
Why this is correct
SMTP on port 25 is used for sending email; blocking it prevents outgoing messages while receiving works.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
HTTPS on port 443
Why it's wrong here
HTTPS is for secure web browsing; blocking it would affect internet access, not email sending specifically.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between incoming (POP3/IMAP) and outgoing (SMTP) mail protocols, trapping candidates who confuse receiving ports with sending ports.
Detailed technical explanation
How to think about this question
SMTP operates over TCP port 25 for unencrypted communication, but many ISPs and firewalls block port 25 to prevent spam relay. Modern email clients often use SMTP with STARTTLS on port 587 (submission) or SMTPS on port 465 for secure sending. The exam focuses on the standard unencrypted SMTP port 25 as the default for sending, which is the most common port blocked in restrictive firewall policies.
KKey Concepts to Remember
- Read the scenario before looking for a memorised answer.
- Find the constraint that changes the correct option.
- Eliminate answers that are true in general but not in this case.
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A practitioner preparing for the 220-1201 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
What to study next
Got this wrong? Here's your next step.
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FAQ
Questions learners often ask
What does this 220-1201 question test?
Network Protocols — This question tests Network Protocols — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: SMTP on port 25 — SMTP (Simple Mail Transfer Protocol) is used for sending outgoing email messages. By default, SMTP uses port 25. If the firewall blocks outbound traffic on port 25, the email client can still receive messages (using POP3 or IMAP) but will be unable to send any, perfectly matching the user's symptoms.
What should I do if I get this 220-1201 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
Same concept, more angles
3 more ways this is tested on 220-1201
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A user reports that their email client cannot send messages but can receive them. The technician verifies that the email server is reachable and that the user's credentials are correct. Which protocol is most likely misconfigured in the email client?
medium- ✓ A.SMTP
- B.POP3
- C.IMAP
- D.HTTP
Why A: SMTP (Simple Mail Transfer Protocol) is used for sending outgoing email messages from a client to a server or between mail servers. Since the user can receive emails but not send them, and the server is reachable with correct credentials, the most likely misconfiguration is in the SMTP settings (e.g., wrong port, missing authentication, or incorrect server address).
Variation 2. 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?
medium- A.Port 25
- ✓ B.Port 110
- C.Port 443
- D.Port 53
Why B: 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.
Variation 3. A customer reports that their new all-in-one printer can scan and copy locally, but they cannot send scanned documents to their email address. The printer is connected to the office network and can ping other devices. Which protocol is most likely misconfigured on the printer?
easy- A.FTP
- B.IMAP
- ✓ C.SMTP
- D.SNMP
Why C: The printer can scan and copy locally and is reachable on the network (ping works), but it cannot send scanned documents to an email address. This indicates the printer is configured to use SMTP (Simple Mail Transfer Protocol) to send the email, but the SMTP server settings (server address, port, authentication, or encryption) are likely incorrect or missing. SMTP is the standard protocol for sending outgoing email, which is exactly what the printer needs to deliver the scanned document as an email attachment.
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Last reviewed: Jul 4, 2026
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