SMTP VRFY Enumeration: How Attackers Verify Email Users
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?
Quick Answer
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.”
⚠ Common exam trap
A common mix-up: candidates confuse VRFY with RCPT TO, but the question explicitly states the VRFY command is used, making 'VRFY enumeration' the only correct answer.
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
✓
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.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
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EXPN enumeration
Why it's wrong here
EXPN expands a mailing list into its member addresses, requiring a list name rather than a single mailbox. It is the correct technique when enumerating distribution list membership, whereas VRFY validates one address at a time.
- ✗
SMTP banner grabbing
Why it's wrong here
SMTP banner grabbing reads the greeting returned on connection, not per-recipient responses to VRFY. The 250/550 replies here are direct VRFY results, so banner grabbing cannot produce them. It is tempting because banner grabbing genuinely reveals server software and version, and would be the right choice when fingerprinting the mail server itself.
- ✗
RCPT TO enumeration
Why it's wrong here
VRFY tests a mailbox by asking the server to confirm the address directly, which is what produced the 250 and 550 replies here. RCPT TO enumeration instead issues MAIL FROM followed by RCPT TO during a message transaction, harvesting responses to map valid recipients; it is the fallback when VRFY is disabled, not the technique shown.
- ✓
VRFY enumeration
Why this is correct
The VRFY command asks the SMTP server to confirm whether a mailbox exists; replies of 250 for admin and 550 for fake reveal valid accounts. This is VRFY enumeration, distinguishing it from EXPN, RCPT TO or EHLO-based address harvesting.
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Same concept, more angles
1 more way this is tested on CEH
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: 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.
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This CEH practice question is part of Courseiva's free EC-Council 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 CEH exam.