I found interesting concept somewhere in the web to reduce the spam that comes in to the organization.
As we all know mx record plays a major role for receiving email. And it is also the entry point for the spam to enter into the organization.
“Nolisting” is a concept of creating more than two mx records, and setting the primary mx record to “nowhere”(ie 0.0.0.0).
As per RFC-(forgot the number), a genuine email from a genuine sender should try the secondary if the primary is not available or invalid.
Spammers won’t strain much to try to resend the email to the secondary mx record. This way you can reduce the spam coming into your organization