Spamhaus.org and open resolvers

Spamhaus does not like open resolvers like Google, Cloudfare and Quad9. By using these they cannot determine if you should use the commercial package. The first 100.000 requests are free but after that you are supposed to have a paid subscription.

In your mail.log you will see something like:

Server error: ‘554 5.7.1 Service unavailable; Sender address [MYEMAIL@HOST.COM] blocked using dbl.spamhaus.org 8; Error: open resolver; DNSBL Error Code - Open/public resolver - The Spamhaus Project

You will find their explanation:

To prevent any issues with your email stream, stop accessing Spamhaus’ free blocklists via the Public Mirrors and start accessing the blocklists via our free Data Query Service (DQS), which you can sign up for here.

Of cause you can do but if you do not like to subscribe to just another service, change your Postfix main.cf line from:

smtpd_recipient_restrictions = reject_rbl_client zen.spamhaus.org,reject_rbl_client dnsbl.sorbs.net,reject_rbl_client bl.spamcop.net .... in smtpd_recipient_restrictions = reject_rbl_client dnsbl.sorbs.net,reject_rbl_client bl.spamcop.net ....