Full Debian Mail Stack Upgrade

My production mail server runs on Debian and every main layer—Postfix, Zimbra and the Kernel—now needs to move to the latest stable releases. All three components must come through the process with complete data integrity, so the job starts with a solid backup plan: at the moment no backup exists and I will rely on you to design, create and test one before any package is touched.

Once the safety net is in place, the work continues with a carefully staged upgrade. Postfix, Zimbra and the Kernel should be brought up to date, followed by migration to the current Debian stable. I expect minimal downtime, full preservation of mailboxes, calendars, contacts, aliases and logs, and a short verification window after each major step to catch issues early.

The assignment is finished when:

• A restorable, tested backup sits both locally and off-site
• Postfix, Zimbra and the Kernel run the newest stable versions supported by Debian
• All services pass functional tests (mail flow, webmail, mobile sync, spam filtering)
• Basic hardening and security scans show no critical findings
• A concise report records every command and configuration change for future reference

SSH access and necessary credentials will be provided. Please outline your approach, key tools you prefer to use (e.g., rsync, Borg, Ansible, Zimbra backup utilities) and an estimated timeline for each phase.