When configuring a new server, a technician requests that an MX record be created in DNS for the new server, but the record was not entered properly. Which of
the following was MOST likely installed that required an MX record to function properly?

A.
Load balancer

B.
FTP server

C.
Firewall DMZ

D.
Mail server

Explanation:
A mail exchanger record (MX record) is a DNS record used by email servers to determine the name of the email server responsible for accepting email for the
recipient’s domain.
For example a user sends an email to recipient@somedomain.com. The sending user’s email server will query the somedomain.com DNS zone for an MX record
for the domain. The MX record will specify the hostname of the email server responsible for accepting email for the somedomain.com domain, for example,
mailserver.somedomain.com. The sending email server will then perform a second DNS query to resolve mailserver.somedomain.com to an IP address. The
sending mail server will then forward the email to the destination mail server.