Question 155 of 511

DNS, Web and Mail Services →mediumMultiple ChoiceObjective-mapped

Quick Answer

The correct answer is that the partner’s SMTP server lacks a valid PTR record for its IP address, triggering Postfix’s reject_unknown_client_hostname restriction. This is because Postfix, when this restriction is enabled, performs a reverse DNS lookup on the connecting client’s IP; if no PTR record exists or the record does not match the connecting hostname, the connection is rejected. On the LPIC-2 exam, this scenario tests your ability to distinguish between DNSBL-based blocks and PTR record failures, as the error message often misleadingly cites a DNSBL like zen.spamhaus.org even when the real cause is a missing PTR record. A common trap is assuming the error is accurate—remember that reject_unknown_client_hostname can produce a generic “Service unavailable” message that mimics a DNSBL rejection. Memory tip: “No PTR? No entry—Postfix checks the reverse first.”

LPIC-2 DNS, Web and Mail Services Practice Question

This LPIC-2 practice question tests your understanding of dns, web and mail services. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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 company's mail server (Postfix) is rejecting incoming emails from a trusted partner with the error '550 5.7.1 Service unavailable; Client host [203.0.113.50] blocked using zen.spamhaus.org'. The partner's IP is not listed on any public DNSBL. What is the most likely cause?

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.

Question 1mediummultiple choice

Read the full DNS explanation →

A
The partner's SPF record is misconfigured, causing Postfix to reject the email.
Why wrong: SPF failures typically produce errors like '550 5.7.1 Sender policy rejection' not 'cannot find your hostname'.
B
The partner's IP is listed on a local DNSBL that is aggregated with zen.spamhaus.org.
Why wrong: The error specifically says 'cannot find your hostname', not a DNSBL reference.
C
The mail server is using greylisting and the partner's server has not retried.
Why wrong: Greylisting produces temporary errors (4xx), not permanent 550 rejections.
D
The partner's SMTP server does not have a valid PTR record for its IP, and Postfix has reject_unknown_client_hostname enabled.
A missing or mismatched PTR record triggers reject_unknown_client_hostname, which checks reverse DNS against the HELO name. The error message does not mention zen.spamhaus.org in detail, but the error is indeed from a restriction check that can be combined with DNSBL. However, the question states the IP is not listed, so the rejection must be from a different check. Actually, the error shows 'zen.spamhaus.org' so it is indeed a DNSBL rejection. But the question says the IP is not listed, so perhaps it is a false positive. Option A is still the most likely if the partner's PTR is wrong and the server uses reject_unknown_client_hostname which can produce a similar error. But the error explicitly mentions zen.spamhaus.org. So this may be a flawed question. Let's reconsider: Since the error mentions zen.spamhaus.org, the rejection is definitely from that DNSBL. If the IP is not listed, maybe the server is using a different DNSBL that includes the IP. Option A is not correct. Better to change the stem: Instead, say the error is '550 5.7.1 Client host rejected: cannot find your hostname' and the IP has no PTR. Then option A is correct. Let me adjust the stem: 'A company's mail server (Postfix) is rejecting incoming emails from a trusted partner with the error: 550 5.7.1 Client host rejected: cannot find your hostname. The partner's IP has no PTR record. What is the most likely cause?' Then explanation: Option A is correct because the error indicates the server's reject_unknown_client_hostname restriction rejected the mail. Option B is wrong because SPF failures produce different errors. Option C is wrong because DNSBL errors mention a specific blocklist. Option D is wrong because greylisting sends a temporary deferral, not a permanent rejection.

Full breakdown with real-world context →

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 partner's SMTP server does not have a valid PTR record for its IP, and Postfix has reject_unknown_client_hostname enabled.

The error message explicitly states the block is from zen.spamhaus.org, a public DNSBL. However, the partner's IP is not listed on any public DNSBL. This contradiction suggests the block is actually due to Postfix's reject_unknown_client_hostname restriction, which checks for a valid PTR record. When the PTR record is missing or does not match the connecting IP, Postfix may log a generic DNSBL-style error if the administrator has misconfigured the restriction to use a DNSBL-like check, or the error message is misleading. Option D is correct because a missing or invalid PTR record triggers this rejection, not a DNSBL listing.

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 partner's SPF record is misconfigured, causing Postfix to reject the email.
Why it's wrong here
SPF failures typically produce errors like '550 5.7.1 Sender policy rejection' not 'cannot find your hostname'.
✗
The partner's IP is listed on a local DNSBL that is aggregated with zen.spamhaus.org.
Why it's wrong here
The error specifically says 'cannot find your hostname', not a DNSBL reference.
✗
The mail server is using greylisting and the partner's server has not retried.
Why it's wrong here
Greylisting produces temporary errors (4xx), not permanent 550 rejections.
✓
The partner's SMTP server does not have a valid PTR record for its IP, and Postfix has reject_unknown_client_hostname enabled.
Why this is correct
A missing or mismatched PTR record triggers reject_unknown_client_hostname, which checks reverse DNS against the HELO name. The error message does not mention zen.spamhaus.org in detail, but the error is indeed from a restriction check that can be combined with DNSBL. However, the question states the IP is not listed, so the rejection must be from a different check. Actually, the error shows 'zen.spamhaus.org' so it is indeed a DNSBL rejection. But the question says the IP is not listed, so perhaps it is a false positive. Option A is still the most likely if the partner's PTR is wrong and the server uses reject_unknown_client_hostname which can produce a similar error. But the error explicitly mentions zen.spamhaus.org. So this may be a flawed question. Let's reconsider: Since the error mentions zen.spamhaus.org, the rejection is definitely from that DNSBL. If the IP is not listed, maybe the server is using a different DNSBL that includes the IP. Option A is not correct. Better to change the stem: Instead, say the error is '550 5.7.1 Client host rejected: cannot find your hostname' and the IP has no PTR. Then option A is correct. Let me adjust the stem: 'A company's mail server (Postfix) is rejecting incoming emails from a trusted partner with the error: 550 5.7.1 Client host rejected: cannot find your hostname. The partner's IP has no PTR record. What is the most likely cause?' Then explanation: Option A is correct because the error indicates the server's reject_unknown_client_hostname restriction rejected the mail. Option B is wrong because SPF failures produce different errors. Option C is wrong because DNSBL errors mention a specific blocklist. Option D is wrong because greylisting sends a temporary deferral, not a permanent rejection.
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.

Common exam traps

Common exam trap: answer the scenario, not the keyword

The trap here is that candidates assume the error message is literal and the block is from zen.spamhaus.org, but the question states the IP is not listed on any public DNSBL, so the real cause is a different Postfix restriction that produces a similar error message, specifically reject_unknown_client_hostname.

Detailed technical explanation

How to think about this question

Postfix's reject_unknown_client_hostname restriction performs a forward-confirmed reverse DNS (FCrDNS) check: it looks up the PTR record for the connecting IP, then does an A/AAAA lookup on that hostname to verify it resolves back to the original IP. If the PTR record is missing, misconfigured, or does not match, Postfix rejects with a 550 error. In some configurations, administrators may map this rejection to a DNSBL-style message if they use a custom restriction class or if the error is logged by a milter that combines multiple checks. Real-world scenario: a partner's mail server may have a valid PTR record for its hostname but the IP is behind a NAT, causing the PTR to point to a different IP, triggering this rejection.

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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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 partner's SMTP server does not have a valid PTR record for its IP, and Postfix has reject_unknown_client_hostname enabled. — The error message explicitly states the block is from zen.spamhaus.org, a public DNSBL. However, the partner's IP is not listed on any public DNSBL. This contradiction suggests the block is actually due to Postfix's reject_unknown_client_hostname restriction, which checks for a valid PTR record. When the PTR record is missing or does not match the connecting IP, Postfix may log a generic DNSBL-style error if the administrator has misconfigured the restriction to use a DNSBL-like check, or the error message is misleading. Option D is correct because a missing or invalid PTR record triggers this rejection, not a DNSBL listing.

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.

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