Is your feature request related to a problem? Please describe.
I tried to apply null MX records on a domain at INWX.
But I get an error that it's not supported (

Describe the solution you'd like
Based on their documentation, we can disable the check for MxNull.

Describe alternatives you've considered
I'm currently using an IGNORE("@", "MX") rule to manage it manually in the UI.

Additional context
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