A mail exchange (MX) record is a Domain Name System (DNS) record that is required to deliver an email to an address. It identifies the mail server in charge of receiving incoming emails for a particular domain and where outgoing mails should be addressed from. In short, it is in charge of email flow. So, if your MX records are not routed to the appropriate location, you will not able to receive or send emails.
One way of ensuring the integrity and reliability of your MX servers is by using Reverse MX API. With it, you can identify all of the domains connected to each of your mail servers. That helps ensure that domain records are updated and servers are correctly configured.