While examining audit logs, you discover that people are able to telnet into the SMTP server on port 25. You would like to block this, though you do not see any evidence of an attack or other wrong do

Blocking port 25 at the firewall would prevent all incoming SMTP traffic, including legitimate email delivery to the server, thus disrupting normal email functionality.

Shutting off the SMTP service would completely disable the email server, preventing all normal email operations and deliveries.

Forcing username and password authentication might not be feasible for standard SMTP inbound connections (e.g., from other mail servers) and does not prevent a Telnet client from *connecting* to port 25, only from performing unauthorized actions.

Switching email server software is a major system change that does not directly address the specific issue of selectively blocking Telnet access while preserving email functionality on the existing server.

Blocking port 25 at the firewall (A) would prevent all legitimate SMTP traffic, not just Telnet. Shutting off the SMTP service (B) would disable all email functionality. Forcing username/password (C) might not be applicable for all inbound SMTP connections and wouldn't stop a Telnet client from connecting. Switching server software (D) is a drastic solution that doesn't solve the specific problem. Since Telnet is merely a client interacting with the SMTP service on port 25, blocking 'Telnet to port 25' without affecting other SMTP clients connecting to the same port is not directly achievable through these simple options, hence 'None of the above' is the best answer.