Quick Answer
The answer is the RCPT TO command. This is correct because SMTP uses the RCPT TO command to define a message’s recipient, and an SMTP server will respond with a distinct status code for valid versus invalid addresses—typically a “250 OK” for a real mailbox and a “550 No such user” for a non-existent one. By systematically sending crafted RCPT TO commands, an attacker can enumerate valid email addresses without needing to send a full email, directly exploiting the protocol’s built-in recipient verification. On the Certified Ethical Hacker CEH exam, this technique tests your understanding of SMTP enumeration as a reconnaissance method; a common trap is confusing it with VRFY enumeration, which uses a different command but achieves a similar goal. Remember the mnemonic: “RCPT TO reveals the recipient truth” — the server’s reply tells you if the address is valid or not.
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 a tool that sends crafted RCPT TO commands to an SMTP server to verify email addresses. Which SMTP enumeration technique is being used?
AUTH
Why wrong: AUTH is used for SMTP authentication, not enumeration.
RCPT TO
Correct. RCPT TO can be used for email address enumeration based on server responses.
EXPN
Why wrong: EXPN expands mailing lists, not individual addresses via RCPT TO.
VRFY
Why wrong: VRFY is a separate SMTP command for verifying addresses, not RCPT TO.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
RCPT TO
The RCPT TO command is used in SMTP to specify the recipient of an email. By sending crafted RCPT TO commands to an SMTP server, an attacker can observe the server's response (e.g., '250 OK' for valid addresses vs. '550 No such user' for invalid ones) to enumerate valid email addresses. This technique directly exploits the SMTP protocol's recipient verification behavior.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
AUTH
Why it's wrong here
AUTH is used for SMTP authentication, not enumeration.
RCPT TO
Why this is correct
Correct. RCPT TO can be used for email address enumeration based on server responses.
Related concept
Read the scenario before looking for a memorised answer.
EXPN
Why it's wrong here
EXPN expands mailing lists, not individual addresses via RCPT TO.
VRFY
Why it's wrong here
VRFY is a separate SMTP command for verifying addresses, not RCPT TO.
Common exam traps
The trap here is that candidates often confuse RCPT TO with VRFY, assuming VRFY is the primary enumeration command, but in practice, VRFY is frequently disabled, making RCPT TO the more reliable and commonly tested technique in CEH scenarios.
Trap categories for this question
Command / output trap
VRFY is a separate SMTP command for verifying addresses, not RCPT TO.
Detailed technical explanation
SMTP servers typically respond with '250 OK' for RCPT TO if the recipient is accepted (even if the mailbox doesn't exist, depending on configuration), but many servers return distinct error codes like '550' for invalid recipients. Attackers can automate this by sending multiple RCPT TO commands in a single SMTP session (per RFC 5321) to build a list of valid addresses, often bypassing rate limits by rotating source IPs or using slow, low-and-slow techniques.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
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.
What to study next
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FAQ
Enumeration and System Hacking — This question tests Enumeration and System Hacking — Read the scenario before looking for a memorised answer..
The correct answer is: RCPT TO — The RCPT TO command is used in SMTP to specify the recipient of an email. By sending crafted RCPT TO commands to an SMTP server, an attacker can observe the server's response (e.g., '250 OK' for valid addresses vs. '550 No such user' for invalid ones) to enumerate valid email addresses. This technique directly exploits the SMTP protocol's recipient verification behavior.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 24, 2026
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