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 few minutes until Zimbra Postfix reload. Or you can also reloading Postfix manually 😉
Good luck and hopefully useful 🙂