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.”
220-1101 Network Protocols Practice Question
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?
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
Email sending typically uses SMTP on port 25 (or port 587 for submission). If the firewall blocks outbound traffic on port 25, the email client cannot send messages, but receiving (using POP3 or IMAP) remains unaffected. This is a common security measure to prevent spam, but it can interfere with legitimate email sending.
Key principle: NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
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
Static NAT maps one inside address to one outside address.
✗
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: NAT rules depend on direction and matching traffic
NAT is not only about the public address. The inside/outside interface roles and the ACL or rule that matches traffic are just as important.
Detailed technical explanation
How to think about this question
NAT questions usually test address translation, overload/PAT behaviour, static mappings and whether the right traffic is being translated. Read the interface direction and address terms carefully.
KKey Concepts to Remember
Static NAT maps one inside address to one outside address.
PAT allows many inside hosts to share one public address using ports.
Inside local and inside global describe the private and translated addresses.
NAT ACLs identify traffic for translation, not always security filtering.
TExam Day Tips
→Identify inside and outside interfaces first.
→Check whether the scenario needs static NAT, dynamic NAT or PAT.
→Do not confuse NAT matching ACLs with normal packet-filtering intent.
Key takeaway
NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
Real-world example
How this comes up in practice
A small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 220-1201 question in full detail.
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related 220-1201 NAT questions on configuration and troubleshooting.
Network Protocols — This question tests Network Protocols — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: SMTP on port 25 — Email sending typically uses SMTP on port 25 (or port 587 for submission). If the firewall blocks outbound traffic on port 25, the email client cannot send messages, but receiving (using POP3 or IMAP) remains unaffected. This is a common security measure to prevent spam, but it can interfere with legitimate email sending.
What should I do if I get this 220-1201 question wrong?
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related 220-1201 NAT questions on configuration and troubleshooting.
What is the key concept behind this question?
Static NAT maps one inside address to one outside address.
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 →
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: This scenario tests knowledge of SMTP, the protocol used for sending email. Receiving email typically uses POP3 or IMAP, so a sending issue points to SMTP configuration.
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: This question tests knowledge of default ports: POP3 uses port 110 (or 995 for SSL). If sending works (SMTP on port 25/587) but receiving fails, the firewall likely blocks port 110 (POP3) or 995.
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 scenario describes a printer that can perform local functions but fails to send scanned documents via email, which typically uses SMTP (Simple Mail Transfer Protocol) for outgoing mail. Since the printer can ping other devices, basic network connectivity is working, pointing to a higher-layer protocol issue. SMTP is the standard protocol for sending email, so its misconfiguration would prevent email delivery.
Last reviewed: Jun 18, 2026
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This 220-1201 practice question is part of Courseiva's free CompTIA certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 220-1201 exam.
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