What is a DNS MX Record?

Do you ever wonder how email is delivered to your domain? It may seem like it’s simply handled by an email client, but there’s more to it than that. In order for messages to be sent to the right place, the server needs to know how and where to deliver them. That’s where Mail Exchange (MX) records come in.

MX DNS Records Explained

The purpose of an MX record is to route emails to a mail server using the Simple Mail Transfer Protocol (SMTP). It’s what tells resolving nameservers where your domain “mailbox” is, and without it, you wouldn’t get your emails. In order for an MX record to function correctly, it must point to an A or AAAA record. 

Tip: MX Records can never be pointed to a CNAME record.

MX Record Example

MX Record Priority

In the above example, you may have noticed that there is a Priority value for MX records. This value indicates which mail server is the preferred server, or which one should be used first. In the case of MX records, the lower value, also known as an MX level, represents the favored server. 

While Failover isn’t compatible with MX records, configuring multiple MX records essentially achieves the same result. If your preferred mail server is unavailable, messages will be directed to an alternate mail server based on priority. If you have MX records with the same priority, they’ll be treated the same as a round robin configuration and your email will be directed to your mail servers randomly. 

Tip: Priorities are most commonly set in increments of 5 or 10, but can also start at 0 and just be single digits, or be say 53 or 102. Many people use increments of 5 or 10 to allow for future adjustments, but it really boils down to your organization’s preferences. The important thing to remember is the lowest value should be assigned to your most preferred server.

DNS Lookup for MX Records

One way to make sure your MX records are correct or that any changes you’ve made to them have been propagated is by performing a DNS lookup. A lookup will list all the MX records for your domain based on the order of the priority values you set in your configuration. 

Here are a few tools that you can use for testing your records:

DNS Lookup Tool by Constellix

MX Toolbox                                               

DNS Lookup

Google Admin Toolbox

MX Record Summary

And there you have it. Once you know what MX records are for, configuring them is relatively simple. It’s always good practice to have at least two mail servers, that way you’ll always receive your email. Whether you prioritize one server over another or want your email cycled to each server randomly depends on the unique needs of your domain. The most important thing to remember is that an MX record must be mapped to an A or AAAA record in order to function properly.