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…

Blocking port 25 at the firewall would drop all inbound SMTP connections, preventing legitimate email from reaching the server.

Shutting off the SMTP service would completely disable email processing on the server, not just unauthorized interactive sessions.

Requiring credentials does not prevent a TCP connection from being established to port 25; it only adds an authentication step after the connection is already open.

Switching from Exchange to Sendmail is irrelevant because both mail server platforms accept connections on port 25 using the same text-based SMTP protocol.

SMTP operates on port 25 using a plain text, command-response protocol that is indistinguishable at the transport layer from a telnet session, so there is no way to block one without blocking the other. Option A would stop all inbound email delivery; Option B would shut down the mail service entirely; Option C adds authentication within the session but does not prevent the TCP connection itself; Option D is irrelevant because both Exchange and Sendmail use port 25 the same way. The real solution - such as restricting connecting IP addresses via an access control list - is not among the choices.