"Null MX" is an internet standard, specifically RFC 7505, to tell others that a domain does not receive e-mail 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 e-mail from employees. But most domain names registered are not used for sending or receiving e-mail.
You may have experienced sending e-mail, and after some time getting strange e-mails about your e-mail not being delivered, and that the system will try again later? Did you know that e-mail systems might try to deliver your e-mail for hours and several days until it gives up, and only then will you be notified that your e-mail could not be delivered.
This can cause delays, misunderstandings and unnecessary "did you get my e-mail 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 e-mail, the domain will clearly state that it doesn't accept any e-mail, period. Anyone attempting to send e-mail to that domain will then immediately received a notification saying you cannot send e-mail to that domain.
This will make our use of e-mail 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!