"Null MX" is an internet standard, specifically RFC 7505, to tell others that a domain does not receive email from anyone.
Many organisations register more than one domain name. As an example we have registered securepractice.no & securepractice.co. Not much of a difference as you can see!
Registering multiple domains are done for different reasons. Brand protection, company names within a large enterprise or operations in multiple countries & markets. You can also find that dedicated domains names are used for sending newsletters, invoices or email from employees. But most domain names registered are not used for sending or receiving email.
You may have experienced sending email, and after some time getting strange emails about your email not being delivered, and that the system will try again later? Did you know that email systems might try to deliver your email for hours and several days until it gives up, and only then will you be notified that your email could not be delivered.
This can cause delays, misunderstandings and unnecessary "did you get my email yet?" phone calls. It creates unnecessary computer traffic, it costs money and a tiny bit of electric power as well.
By creating a NULL MX RECORD for a domain name which isn't meant to receive email, the domain will clearly state that it doesn't accept any email, period. Anyone attempting to send email to that domain will then immediately received a notification saying you cannot send email to that domain.
This will make our use of email a little more effective, avoid misunderstandings related to delayed mail, and perhaps save a tiny bit of power as well.
How to create a NULL MX record?
It is actually very simple! If we no longer wanted to receive email using our domain securepractice.no, all we would need is a single line of text.
Here known as a NULL MX record, which is to be put into DNS for our domain name, this is all we need:
securepractice.no 0 .
Not much typing to do, as you can see!