Answer

PROBLEM

Inbound emails are blocked after the DNSBL option has been enabled.

ENVIRONMENT

• Microsoft Exchange
• All supported environments

SOLUTION

Please ensure that only operational blocklists are configured and listed under the filter properties.

CAUSE

This issue will occur if using a DNS blocklist which has ceased to operate and answers all requests with a positive result. If using a non-existent list, all emails which are not whitelisted will be blocked.