The correct action is to use the EXPN command to expand mailing lists and verify addresses. This works because the SMTP VRFY command, which checks if a specific user exists, is often restricted by modern mail servers; a response like '252 Cannot VRFY user, but will accept message' indicates that VRFY is disabled or deliberately obfuscated, but the server still supports the EXPN command. EXPN reveals the actual members of a mailing list or alias, allowing you to enumerate valid email addresses indirectly by querying known or guessed list names. On the Certified Ethical Hacker CEH exam, this scenario tests your understanding of SMTP enumeration techniques and the distinction between VRFY and EXPN—a common trap is assuming VRFY is the only option when it fails. Remember the memory tip: "If VRFY is denied, EXPN is your guide to who's inside."
CEH Enumeration and System Hacking Practice Question
This CEH practice question tests your understanding of enumeration and system hacking. 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.
During a penetration test, a tester uses the SMTP VRFY command against a mail server. The server responds with '252 Cannot VRFY user, but will accept message' for most usernames. Which action should the tester take to enumerate valid email addresses more effectively?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Use the EXPN command to expand mailing lists and verify addresses
The SMTP VRFY command is used to verify whether a specific user exists on the mail server. When the server responds with '252 Cannot VRFY user, but will accept message', it indicates that VRFY is disabled or restricted, but the server still supports the EXPN command. EXPN (Expand) reveals the actual members of a mailing list or alias, allowing the tester to enumerate valid email addresses indirectly by querying known or guessed list names.
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.
✓
Use the EXPN command to expand mailing lists and verify addresses
Why this is correct
EXPN can reveal valid addresses from mailing lists; RCPT TO is another alternative.
Related concept
Read the scenario before looking for a memorised answer.
✗
Use the NetBIOS nbtstat command to query the mail server
Why it's wrong here
NetBIOS enumeration does not provide email addresses.
✗
Run a brute-force attack on the SMTP service using Hydra
Why it's wrong here
Brute-force attacks target passwords, not email address enumeration.
✗
Switch to SNMP enumeration to gather email addresses
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates assume a '252' response to VRFY means the user does not exist, when in fact it indicates VRFY is disabled, and they overlook the EXPN command as an alternative enumeration method.
Detailed technical explanation
How to think about this question
EXPN is defined in RFC 5321 and allows a client to request the expansion of a mailing list or alias, returning the individual mailbox addresses. For example, sending 'EXPN staff' might return 'alice@example.com' and 'bob@example.com'. Many mail servers disable VRFY for privacy but leave EXPN enabled for operational reasons, making it a valuable enumeration tool during reconnaissance.
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 CEH 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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this CEH question in full detail.
Enumeration and System Hacking — This question tests Enumeration and System Hacking — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Use the EXPN command to expand mailing lists and verify addresses — The SMTP VRFY command is used to verify whether a specific user exists on the mail server. When the server responds with '252 Cannot VRFY user, but will accept message', it indicates that VRFY is disabled or restricted, but the server still supports the EXPN command. EXPN (Expand) reveals the actual members of a mailing list or alias, allowing the tester to enumerate valid email addresses indirectly by querying known or guessed list names.
What should I do if I get this CEH 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 →
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. Which TWO of the following are valid SMTP enumeration commands that can be used to discover valid email addresses? (Select 2)
medium
A.HELO
B.RCPT TO
✓ C.EXPN
D.ATRN
✓ E.VRFY
Why C: EXPN is a valid SMTP command that requests the server to expand a mailing list or alias, revealing the individual email addresses that belong to it. This enumeration technique is useful for discovering valid email addresses without sending a message, as defined in RFC 821.
Variation 2. A penetration tester uses the SMTP commands VRFY and EXPN on a mail server. What is the tester MOST likely trying to accomplish?
medium
✓ A.To enumerate valid email addresses and distribution lists
B.To extract email content from the server
C.To perform a mail relay attack
D.To test for open relay
Why A: The VRFY command asks the mail server to verify whether a given email address exists, while EXPN requests the members of a mailing list or alias. By issuing these commands, the tester can enumerate valid user accounts and distribution lists on the server, which is a key step in building a target list for further attacks such as password guessing or phishing.
Last reviewed: Jun 24, 2026
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