An Nmap SMTP enumeration script returns valid usernames.

This scenario demonstrates SMTP Enumeration, a classic enumeration technique described in CEH v13 Network Enumeration. SMTP servers may expose commands such as VRFY, EXPN, and RCPT TO, which can be abused to validate user accounts.

When these commands are enabled without restriction, attackers can enumerate valid usernames without authentication. The Nmap script used explicitly leverages these commands, confirming the misconfiguration.

Option B is incorrect because authentication is not bypassed. Option C concerns DNS routing, not user enumeration. Option D relates to encryption, not enumeration capability.

CEH v13 strongly recommends disabling or restricting SMTP user verification commands. Thus, Option A is correct.