You are the Exchange administrator for Contoso, Ltd. The company has a business partnership with Trey Research. Each company has its own Active Directory domain.
The domains are named contoso.com and treyresearch.com, respectively. Contoso Ltd., and Trey Research are in the same Exchange organization.
The organization contains three servers that run Exchange Server 2003, One Exchange server is configured with an SMTP connector for all Internet e-mail.
Most users have SMTP addresses of alias@contoso.com. However, some users have SMTP addresses of alias@treyresearch.com.
The alias@treyresearch.com users report that they cannot receive e-mail messages from the internet. However, they can send and receive e-mail messages internally.
They can also send e-mail messages to Internet recipients.
You need to ensure that all users can send and receive Internet e-mail messages.
What should you do?

A.
Create a recipient policy that adds the alias@treyresearch.com SMTP address for all Trey Research users.

B.
Add the user principal name (UPN) suffix for trey treyresearch.com to the forest.

C.
Add the treyresearch.com namespace to the SMTP connector at contoso.com.

D.
Add a mail exchanger (MX) record to the treyresearch.com domain on the appropriate DNS servers.

Explanation:
All mail is flowing correctly with the exception of inbound mail for treyresearch.com.
The only possible explanation for this is that the external DNS servers do not know how
to handle incoming mail for this domain. The way to resolve this is to add an MX record
to the external DNS server for the treyresearch.com domain.
Incorrect Answers:
A. Adding alias@treyresearch.com to the recipient policy will add another SMTP address to the list of possible mail addresses.
This will not allow users to receive mail on that address. Even if it did, the answer would still be incorrect since only a few users are using Treyresearch,
and these users already have this as an SMTP address.
B. Adding a UPN suffix will not affect e-mail flow in any way. It is used to help streamline domain naming in a forest. Therefore, this can’t be the correct answer.
C. Adding the treyresearch namespace to the SMTP connector will not resolve the problem,
as the SMTP connector is used only for connections between sites, and has no effect on incoming e-mail from outside the organization.