The answer is that the SMTP server is revealing valid usernames via the VRFY command. This is correct because the VRFY command, defined in RFC 821, asks the mail server to confirm whether a specific mailbox exists; a '252' response indicates a valid user, while a '550 5.1.1' response means the user is unknown. By comparing these distinct replies, an attacker can perform SMTP VRFY user enumeration to build a list of legitimate accounts on the target system. On the Certified Ethical Hacker CEH exam, this technique tests your understanding of information gathering and reconnaissance through mail server misconfigurations, often appearing in questions about footprinting or service enumeration. A common trap is confusing the VRFY command with EXPN (which expands mailing lists) or assuming all non-delivery codes are identical. Memory tip: think "252 = Valid, 550 = Unknown" — the first digit '2' signals success, while '5' signals failure, making the distinction easy to recall.
CEH Enumeration and System Hacking Practice Question
This CEH practice question tests your understanding of enumeration and system hacking. Examine the command output carefully: the correct answer depends on what the output actually shows, not on general recall alone. 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, you run `smtp-user-enum -M VRFY -U users.txt -t 10.0.0.10` and receive responses '252 2.5.2 User <username>' for some users and '550 5.1.1 User unknown' for others. What does this indicate?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The SMTP server is revealing valid usernames via the VRFY command
The VRFY command in SMTP (RFC 821) asks the server to verify whether a mailbox exists. A response starting with '252' indicates the user is valid, while '550 5.1.1' means the user does not exist. By comparing these responses, the attacker can enumerate valid usernames on the mail server, which is exactly what the output shows.
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 SMTP server is using EXPN instead of VRFY
Why it's wrong here
EXPN is for expanding mailing lists; the output shows VRFY responses.
✓
The SMTP server is revealing valid usernames via the VRFY command
Why this is correct
The differing responses allow an attacker to identify valid users, which is a security issue.
Related concept
Read the scenario before looking for a memorised answer.
✗
The SMTP server is vulnerable to command injection
Why it's wrong here
VRFY is a standard SMTP command; its response difference indicates user enumeration, not injection.
✗
The SMTP server is properly configured to prevent enumeration
Why it's wrong here
A proper configuration would return a generic response for both valid and invalid users.
Common exam traps
Common exam trap: answer the scenario, not the keyword
EC-Council often tests the distinction between VRFY and EXPN, where candidates mistakenly think EXPN is being used when the output clearly shows VRFY responses, or they assume any non-250 response means the server is secure.
Trap categories for this question
Command / output trap
EXPN is for expanding mailing lists; the output shows VRFY responses.
Detailed technical explanation
How to think about this question
The VRFY command is defined in RFC 821 and typically returns code 250 for a valid user and 550 for an unknown user, but some servers use 252 as a 'cannot verify but will try to deliver' response, which still leaks existence. In real-world assessments, disabling VRFY in the SMTP daemon (e.g., by setting 'disable_vrfy_command = yes' in Postfix) is a common hardening step to prevent this exact enumeration technique.
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: The SMTP server is revealing valid usernames via the VRFY command — The VRFY command in SMTP (RFC 821) asks the server to verify whether a mailbox exists. A response starting with '252' indicates the user is valid, while '550 5.1.1' means the user does not exist. By comparing these responses, the attacker can enumerate valid usernames on the mail server, which is exactly what the output shows.
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 enumeration technique would be MOST effective for gathering usernames from an SMTP server that supports the VRFY command?
easy
A.SNMP enumeration using SNMPwalk
✓ B.SMTP VRFY enumeration
C.NetBIOS enumeration using nbtstat
D.LDAP anonymous queries
Why B: The VRFY command asks the server to verify a username. Attackers can use it to enumerate valid accounts.
Last reviewed: Jun 30, 2026
Question Discussion
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