Fix Dovecot & Postfix Permissions

My Webmin / Virtualmin mail server was running smoothly until I had to reinstall Postfix. Since then, messages are bounced or never show up because the two core services are tripping over file-system restrictions:

• Dovecot is throwing file and folder permission errors whenever it tries to reach users’ maildirs.
• Postfix will no longer accept incoming mail, clearly a permission-related failure.

I need a clean overhaul of user, group and directory rights so that both services can coexist again without rejecting or losing mail. The job is finished when:

1. Dovecot reads and writes every mailbox without permission warnings in the logs.
2. Postfix reliably receives external messages and hands them off to Dovecot.
3. Test messages in and out complete successfully, with logs free of permission or authentication errors.
4. A brief note describes which users, groups and paths were adjusted so I can reproduce the fix on future updates.
5. Obviously emails must be sent and received
6. I want to move two of the accounts away to new servers so they have their own IP address. I've setup the one, but one is still using a floating IP on the primary server.

Everything runs under Webmin/Virtualmin, so please keep the changes compatible with its expected directory structure. SSH access is ready as soon as we agree on a time slot.