Postfix

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 ...

Dovecot new version 2.4 is a breaking change from 2.3 and deliberately so. With version 2.4 the paid Pro version does have all functionality and some from 2.3. The new Community Edition is only usable for a single server without high availability or cluster functionality. To make sure you break everything the config files from 2.3 and 2.4 are not (backward) compatible. And to make this move even more profitable you will not find a pricing scheme anywhere, you have to ask a quotation for your specific configuration. ...