The answer is VRFY enumeration. This technique is correct because the SMTP VRFY command, defined in RFC 821, directly asks the server to confirm whether a specific email address exists, and the distinct responses—250 OK for valid users and 550 No such user for invalid ones—allow an attacker to systematically verify email accounts without sending a message. On the Certified Ethical Hacker CEH exam, this tests your understanding of service-specific enumeration, often appearing as a scenario where an attacker probes an SMTP server to map valid users for later phishing or password attacks. A common trap is confusing VRFY with EXPN (which expands mailing lists) or RCPT TO (which verifies during mail delivery); remember that VRFY is the explicit “verify” command. Memory tip: think “VRFY = Verify directly, no mail needed.”
CEH Enumeration and System Hacking Practice Question
This CEH practice question tests your understanding of enumeration and system hacking. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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.
An attacker uses the VRFY command on an SMTP server to check the existence of email addresses. The server responds with '250 OK' for 'admin@company.com' and '550 No such user' for 'fake@company.com'. Which SMTP enumeration technique is being used?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
VRFY enumeration
The VRFY command is an SMTP command defined in RFC 821 that asks the server to verify whether a given email address exists. When the server responds with '250 OK' for a valid address and '550 No such user' for an invalid one, the attacker is directly using the VRFY command to enumerate valid users. This is explicitly known as VRFY enumeration.
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.
✗
EXPN enumeration
Why it's wrong here
EXPN is used to expand mailing lists, not verify individual users.
✗
SMTP banner grabbing
Why it's wrong here
Banner grabbing identifies the SMTP server version, not user enumeration.
✗
RCPT TO enumeration
Why it's wrong here
RCPT TO is used during mail delivery to verify recipients, but the question specifically mentions VRFY.
✓
VRFY enumeration
Why this is correct
The VRFY command verifies whether a mailbox exists, and the response codes confirm this technique.
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 confuse VRFY with RCPT TO, but the question explicitly states the VRFY command is used, making 'VRFY enumeration' the only correct answer.
Detailed technical explanation
How to think about this question
Under the hood, the VRFY command is processed by the SMTP server's MTA (Mail Transfer Agent) which checks its local user database or alias maps. In modern SMTP servers, VRFY is often disabled or restricted to prevent user enumeration, as it poses a privacy risk. A real-world scenario is a penetration tester using VRFY against a misconfigured Exchange or Postfix server to build a list of valid employee emails for a phishing campaign.
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: VRFY enumeration — The VRFY command is an SMTP command defined in RFC 821 that asks the server to verify whether a given email address exists. When the server responds with '250 OK' for a valid address and '550 No such user' for an invalid one, the attacker is directly using the VRFY command to enumerate valid users. This is explicitly known as VRFY enumeration.
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
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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. An attacker attempts to enumerate valid email users by connecting to an SMTP server and issuing the following commands: EHLO example.com, VRFY root, VRFY admin, VRFY user1. Which SMTP enumeration technique is being used?
medium
A.RCPT TO
B.MAIL FROM
C.EXPN
✓ D.VRFY
Why D: Option D is correct because the VRFY command is specifically designed to verify whether a mailbox exists on an SMTP server. By issuing VRFY followed by usernames (root, admin, user1), the attacker can enumerate valid email users based on the server's responses (e.g., 250 or 251 for valid, 550 for invalid). This is a classic SMTP user enumeration technique.
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Question Discussion
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