SMTP enumeration involves probing a mail server to gather information about users and configurations. Two important SMTP commands used in enumeration are:

VRFY: Verifies whether a particular email address or user ID exists on the mail server.

EXPN: Reveals the actual addresses behind a mailing list or alias.

These commands allow attackers or penetration testers to enumerate valid user accounts or group memberships, which can later be targeted for phishing, spam, or brute-force attacks.

Incorrect Options:

B. Daily outgoing message limits are not typically disclosed via SMTP.

C. RCPT TO is used to designate a message recipient, but it doesn't enumerate open ports.

D. Mail proxy addresses are not revealed directly via SMTP enumeration.

Reference – CEH v13 Official Courseware:

Module 04: Enumeration

Section: “SMTP Enumeration Techniques”

Tool Reference: smtp-user-enum, Netcat

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