What's an "MX Record"?

Unfortunately, MX records are one of the most misunderstood points of email transmission. Padawan mail admins have many misconceptions. Here are a few:

1. That the MX records have a specific naming convention like mail.domain.com or smtp.domain.com

2. That every server that handles email must be in the MX record

3. That the destination MTA must be in the MX record

4. That an "MX server" is another name for a mail server

5. That a "reverse DNS" lookup queries the sending domain's MX

Now that we know a few of the things an MX record is NOT, let's take a look at what it IS.

An MX record is simply an entry in a domains DNS zone. Typically they consist of one or more "A" type records (which then require resolution) with priorities assigned to them. Also, as with all DNS record a TTL value, or Time To Live, is assigned to them as a whole (and to each type A record as well).

They are simply references to the border MTA(s) responsible for relaying incoming email for a domain. When an email server is asked to email user@domain.com, it queries the DNS servers responsible for the domain (often by recursing to the root name servers looking for SOA records) and asks for the MX records. Typically, the MX records consist of type A records so then another query is given to find the IP addresses associated to them.

Let's take this story as an example. Let's say you're at a friends house discussing a long lost friend.

Friend: Hey I bumped into John last week at the mall
You: Really, I haven't seen him in years! Did you get his phone number?
Friend: Yes, but I don't remember it. You'll have to look in my phone.
You: Where's your phone? (recursive DNS query to the root NS)
Friend: Over on the table
You: What's his entry? (MX record query)
Friend: It's "John Doe"
You open the phone and look at "John Doe's" entry (type A record lookup) and call him on your phone. (transmission)

This simple story, that has probably never actually happened to any of us but should be able to be understood by most of us, is as simple as MX gets. Of course there's more to it (TTL values, "border MTAs", lots more DNS lookups, and any number of issues routing), but those are really more relegated to the realm of troubleshooting, and not something to worry our pretty little heads with right now :-)

Once it's been determined where to send the email, the transmission ensues and sooner or later after being relayed, archived, scanned for spam/viruses, and many more operations the email will eventually find it's home at the destination server and be picked up by the client to be read by the user.

Just think of all of the servers across the globe that played a part in getting your email to it's destination sometime when you're sending email. It really is amazing how it all comes together so seamlessly.

The next time you get the call from your user wondering why it took five seconds to get an email try explaining this to them. Boggle their mind a little bit ;-)


-TEA