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.
- ✗
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.
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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.