Internet SMTP server configured to use DNS

My network consists of a single Active

Directory domain domain.com that contains two

domain contollers. each domain controller runs

Microsoft Windows Server 2003 and is configured

as a DNS server. Each DNS server is configured

with root hints for resolving Internet host

names

The Exchange organisation contains two server

that run Exchange Server 2003. One Exchange

server is configured with two network adapters

and two SMTP virtual servers. One SMTP virtual

server is configured for internet e- mail

messages. The Internet SMTP virtual serveris

configured to use a DNS server at an ISP as an

external DNS server

the firewall configuration for domain.com is

modified to permit only domain controllers to

make DNS queries to the Internet.User report

that they can no longer send e-mail messages to

recipient on the  internet.However, they can

receive e-mail messages from the Internet

I need to ensure that users can use the

Internet to send and receive e-mail messages?

The manager told I can try either of the suitable methods:-

A.Reconfigure the network adapter used by the

internet SMTP virtual server to use the  DNS

server at the ISP

B.Reconfigure the Internet SMTP virtual server

to not use an external DNS server

C. Configure the Internet SMTP virtual server to

use a smart host to deliver e-mail messages. Use

the fully qualified domain name (FQDN) of an

SMTP server managed by the ISP as the smart host

D. Configure the SMTP connector to use a smart

host to e-mail messages.Use the fully qualified

domain name (FQDN) of an SMTP server managed by

the ISP as the smart host.