- A
The server's firewall is blocking outbound connections on port 25, causing connection timeouts.
Why wrong: Blocking port 25 would cause immediate connection failures, not deferral after acceptance.
- B
The DNS resolver on the server is unable to resolve the MX records for the recipient domains.
Postfix defers mail when it cannot resolve the MX or A record for the destination.
- C
The SPF record for the sender domain is missing, causing the recipient's mail server to reject the email.
Why wrong: SPF failure would cause rejection, not deferral, and would appear in logs.
- D
The server's IP address has no PTR record, causing the recipient's mail server to reject the connection.
Why wrong: PTR records are not checked during initial delivery; deferral would not occur.
Quick Answer
The answer is that the DNS resolver on the server is unable to resolve the MX records for the recipient domains. This is correct because Postfix accepts messages into its queue immediately but defers delivery when it cannot perform the necessary DNS MX resolution to find the destination mail exchanger for the recipient’s domain. The mail log shows no errors because deferral is a standard queue management action, not a hard failure, and the server’s resources are fine. On the LPIC-2 exam, this scenario tests your understanding of Postfix’s mail flow and the critical role of DNS in mail routing—a common trap is assuming a log error must appear, when in fact deferred mail often stems from silent DNS failures. Remember the mnemonic: “No MX, no next hop—deferred mail won’t stop.”
LPIC-2 DNS, Web and Mail Services Practice Question
This LPIC-2 practice question tests your understanding of dns, web and mail services. 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 small company runs a LAMP stack web server with Postfix mail server on a single Ubuntu 22.04 instance. The web server hosts a PHP application that sends password reset emails via the local mail server using PHP's mail() function. Recently, users report that password reset emails are not arriving. The administrator checks the mail log and finds that messages are being accepted by Postfix but are not being delivered. The mail queue shows messages with the status 'deferred'. There are no obvious errors in the mail log. The server has sufficient disk space and memory. The administrator suspects a DNS resolution issue. Which of the following is the most likely cause of the deferred mail?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue:
"most likely"Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
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
The DNS resolver on the server is unable to resolve the MX records for the recipient domains.
Postfix accepts the messages but defers delivery when it cannot resolve the recipient domain's MX record. The mail log shows no errors because the deferral is a normal queue action, not a failure. DNS resolution failure for MX records is a common cause of deferred mail, as Postfix cannot determine where to deliver the message.
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.
- ✗
The server's firewall is blocking outbound connections on port 25, causing connection timeouts.
Why it's wrong here
Blocking port 25 would cause immediate connection failures, not deferral after acceptance.
- ✓
The DNS resolver on the server is unable to resolve the MX records for the recipient domains.
Why this is correct
Postfix defers mail when it cannot resolve the MX or A record for the destination.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
The SPF record for the sender domain is missing, causing the recipient's mail server to reject the email.
Why it's wrong here
SPF failure would cause rejection, not deferral, and would appear in logs.
- ✗
The server's IP address has no PTR record, causing the recipient's mail server to reject the connection.
Why it's wrong here
PTR records are not checked during initial delivery; deferral would not occur.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often assume a firewall or network issue is the cause when mail is deferred, but the absence of error logs and the 'deferred' status point specifically to a DNS resolution problem, not a connectivity or authentication issue.
Detailed technical explanation
How to think about this question
Postfix uses the system's DNS resolver (via libc resolver or its own DNS lookups) to query MX records for recipient domains. If the resolver fails (e.g., due to misconfigured /etc/resolv.conf, upstream DNS server issues, or DNSSEC validation failures), Postfix cannot obtain the destination mail exchanger's IP address and defers the message. The `deferred` status in the mail queue indicates a temporary delivery failure, and Postfix will retry based on the `maximal_queue_lifetime` parameter (default 5 days).
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 LPIC-2 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.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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DNS, Web and Mail Services — study guide chapter
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FAQ
Questions learners often ask
What does this LPIC-2 question test?
DNS, Web and Mail Services — This question tests DNS, Web and Mail Services — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The DNS resolver on the server is unable to resolve the MX records for the recipient domains. — Postfix accepts the messages but defers delivery when it cannot resolve the recipient domain's MX record. The mail log shows no errors because the deferral is a normal queue action, not a failure. DNS resolution failure for MX records is a common cause of deferred mail, as Postfix cannot determine where to deliver the message.
What should I do if I get this LPIC-2 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
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 →
Last reviewed: Jun 11, 2026
This LPIC-2 practice question is part of Courseiva's free LPI 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 LPIC-2 exam.
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