An MX Record identifies the email server(s) responsible for a domain name. When sending an email to firstname.lastname@example.org, your email server must first look up the MX Record for xyz.com to see which email server actually handles email for xyz.com (this could be mail.xyz.com or someone else's email server like mail.isp.com). Then it looks up the A Record for the email server to connect to its IP address.
An MX Record has a Preference number, indicating the order in which the email server should be used. Email servers will attempt to deliver email to the server with the lowest preference number first, and if unsuccessful continue with the next lowest and so on.