Q: What is a MX Record?
A: MX Record or its full name Mail Exchange Record, is a record of a domain's email server address.

Q: Why do I need to know an email address MX record?
A: Well, there's a number of reasons including ensuring that the emails are being configured to go to the correct mail server. Other purpose this could be useful is to help determine the email server's hosting/email security preference. ie. are they on a hosted email service and are they using a hosted email security provider