I just had to shut down postfix on our secondary mail servers managed by virtualmin GPL out of Virtualmin Pro site-servers using virtualmin mail servers clustering.
Reason was that we had hundreds of smtp processes and thousands of email attempts to wrong emails but in valid domains, making our secondary servers spamming bouncebacks.
Virtualmin really needs to sync valid email addresses, and not only domains, so that secondary mail servers can directly reject invalid mails instead of trying to forward to main mail server and then to generate a SPAM mail bouncing back the original message.
As I understand from another few-months-old ticket in here (searched for it but didn't find) postfix supports a "valid emails" file, so that should be implementable.
We can't switch secondary mail servers on before that is resolved. Not a huge issue as our main servers are High-Availability, but still an issue, classifying it as a bug, as it makes the feature unusable.