==== REQUIRED BASIC INFO OF YOUR IREDMAIL SERVER ====
- iRedMail version (check /etc/iredmail-release): 1.6.7
- Deployed with iRedMail Easy or the downloadable installer? no
- Linux/BSD distribution name and version: Debian 12.4
- Store mail accounts in which backend (LDAP/MySQL/PGSQL): LDAP
- Web server (Apache or Nginx): Nginx
- Manage mail accounts with iRedAdmin-Pro? no
- [IMPORTANT] Related original log or error message is required if you're experiencing an issue.
====
Goal
Build a fail-over cluster with 4 servers (2 backend servers behind HAProxy + KeepAlived).
Requirements
•    A valid mail domain name. We use dinhdu.click  as mail domain name in this document.
•    4 servers, all are Debian 12.4.
o    2 servers run HAProxy + KeepAlived as a frontend for load-balance (HAProxy) and fail-over (KeepAlived).
o    2 servers run the actual mail services. We will install the latest iRedMail 1.6.7 release for this.
Options:
o    1 servers run Opnsense 23.7 as a firewall.