From the following options choose how best you can achieve this objective?

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 doing. However, you are concerned about affecting the normal functionality of the email server. From the following options choose how best you can achieve this objective?

A.
Block port 25 at the firewall

B.
Shut off the SMTP service on the server

C.
Force all connections to use a username and password

D.
None of the above

E.
Switch from Windows Exchange to UNIX Sendmail

Explanation:
Blocking port 25 in the firewall or forcing all connections to use username and password would have the consequences that the server is unable to communicate with other SMTP servers. Turning of the SMTP service would disable the email function completely. All email servers use SMTP to communicate with other email servers and therefore changing email server will not help.