Your network contains an SMTP server.
You discover that the server has two SMTP Virtual Servers named SMTP1 and SMTP2.
SMTP1 starts and SMTP2 stops.
You attempt to start SMTP2 and receive the following error message.

You need to ensure that you can start SMTP2.

Which settings should you modify on SMTP2?

A.
Access control

B.
Advanced Delivery

C.
Connection Control

D.
IP address and TCP port

Explanation:
IP address/TCP port: All unassigned/25. You can use the General tab in the SMTP virtual server properties dialog box to change this setting. If you change this setting, you must specify an IP address/TCP port combination that is not being used by another SMTP virtual server. TCP port 25 is both the default TCP port and the recommended TCP port. More than one virtual server can use the same TCP port, provided that they are configured with different IP addresses. If you do not set a unique IP address/TCP port combination, the SMTP virtual server will not start.

Source: http://technet.microsoft.com/es-es/library/cc758440(WS.10).aspx