What are MX Records? Anatomy of a DNS Zone File – Part Three


A mail exchange record or MX record is a type of record in a DNS zone file. MX records are responsible for specifying which mail server is in charge of receiving email messages on behalf of a domain.

When you send an e-mail, your computer queries the DNS for the MX records of the recipient’s domain name.  This query results in a list of all available MX records that are responsible for accepting incoming mail.  The email client then attempts to make an SMTP (Simple Mail Transfer Protocol) connection to the primary mail server.

MX records are ranked by priority from lowest to highest to specify which mail server is the primary one.  The primary server is always the one to be used, unless it is unavailable and then, the next subsequent server is tried.  If there are no MX records available, the server will simply request the A record of the domain.

I used the following dig query to find the MX records for no-ip.com: dig no-ip.com MX +short (the +short condenses the reply to only show the MX records)

dig no-ip.com MX +short

10 mail1.no-ip.com.
15 mail2.no-ip.com.
5 mail0.no-ip.com.

The reply shows that there are three MX records for no-ip.com.

The primary mail server that email will be routed to is mail0.no-ip.com.  If that server is unavailable, the mail client will then try mail1.no-ip.com, lastly, if that server is unavailable too, it will try mail2.no-ip.com.

To increase reliability, if there are multiple MX records with the same preference number, all of these must be tried first, before the email client can try the next subsequent record. The SMTP client will try all of the MX records until delivery is successful.

Multiple MX records make primary and backup mail possible, but what if you don’t want the headache of running multiple mail servers, but still want the redundancy? No-IP has two easy solution.

Backup MX gives you mail redundancy on exclusive networks with professional mail server admins monitoring the system. Not only that, but No-IP’s Backup MX servers filter viruses and employ RBL lists to help keep the junk from reaching your inbox. No-IP also allows you to provide a valid user list so that bad mail can be stopped at the SMTP level, saving connections and resources on your server.

Another No-IP product, Mail Reflector will help you get around Port 25 if your ISP blocks it.  Mail is first delivered on the standard port to our mail system and then is immediately delivered to your mail server on whichever port you specify. This gives you complete control of your mail system and bypasses seemingly insurmountable restrictions. Mail Reflector also provides all of the features of Backup MX, including virus filtering, greylisting, and customizable RBL lists, so that even if your mail server is temporarily unavailable, mail will not be lost.

Backup MX or Mail Reflector can save you time, money and unnecessary headaches.

Questions on MX records or our mail services? Leave them below!

Check out Part One and Part Two of this series:

The Anatomy of a DNS Zone File: SOA Record

The Anatomy of a DNS Zone File – Part Two: What are NS Records and Why Are They Important to DNS?