Proxmox Mail Gateway- still running on debian etch?

The basis is Etch but all important packages are updated (clamav, postfix, etc.). In order to get very latest version you should apply all hot-fixes, available for commercial licenses only.

See the news about hotfixes, http://www.proxmox.com/news/news-archive

If you have pre-sales questions about the mail gateway you can also contact the support department via https://my.proxmox.com