Most of spam email sent from public IP who did not have PTR/Reverse DNS. What is PTR/Reverse DNS, you can find more information from Wikipedia 😀 : https://en.wikipedia.org/wiki/Reverse_DNS_lookup If you have Zimbra and want to enable PTR/Reverse DNS lookup for incoming, you can perform the following command su – zimbra zmprov mcf +zimbraMtaRestriction "reject_unknown_client_hostname" Please wait ..

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