Your company has a single active directory domain named contoso.com. all servers run windows server 2008. You have a public DNS server named server1, and an e-mail server named server2. Client computers outside the company domain are unable to send e-mail messages to contoso.com. you verify that the host (A) DNS record for server2 is available to external client computers. You need to ensure server2 can receive e-mail messages from external client computers. How should you configure the contoso.com DNS zone?（）

Add a mail exchange (MX) record for server2.
B. Add a mailbox (MB) record for server2. Set the mailbox host setting to server2.
C. Add a canonical (CNAME) record that maps server2 to contoso.com.
D. Add a server locator (SRV) record for server2. Set the service field to _smtp. Set the protocol field to _tcp. Set the port number to 25.