Congratulations! Very interesting project ! I have been running my own servers with ISPConfig with PostFix and Dovecot for many years, but this modern all-in-one solution seems better integrated and complete. As others have commented, rspamd integration would be a great addition, although I think using Proxmox Mail Gateway instead would give more control with more ease. Proxmox doesn't need any special integration. It works like a firewall for mail.