Your network contains an Active Directory forest. The forest contains two domains named
contoso.com and wingtiptoys.com. The network contains a public DNS server named Server1 and an
e-mail server named Server2. You notice that Internet hosts can send e-mail messages to users that
have the @contoso.com SMTP suffix, but cannot send e-mail messages to users that have the
@wingtiptoys.com SMTP suffix. You need to ensure that every user can receive e-mail messages
from the Internet. Which type of resource record should you add to the wingtiptoys.com zone?

A.
Host (A)

B.
Mail Group (MG)

C.
Mail Exchanger (MX)

D.
Service Locator (SRV)

Explanation:
http://technet.microsoft.com/en-us/library/cc958958.aspx