If no MX record is stored in the name server for a domain, but mail is sent to the domain, the mail server usually tries to deliver the mails via the IPv4 address of the A record.
This behavior is not always desired. To prevent this there is the Null-MX entry. This entry tells the mail server that the delivery of e-mails is undesired.
Of course you have the possibility to store such an entry for your domain in our name server.
This can be done as follows in your INWX customer area via the menu item Nameserver:
1. click on the eye symbol
to display the DNS entries of your domain
1. click on Add DNS entry
2. leave the field Name empty
3. select MX as type
4. enter a dot in the Value field
5. in the field Prio enter the priority 0
5. finally click on Save
The Null-MX entry is now finally configured.
For further information, please refer to the following links: