Proxmox Mail Gateway is an email security product that sits as a mail proxy between the firewall and the internal mail server. It filters incoming and outgoing email for spam, viruses, trojans and phishing, using components such as SpamAssassin and ClamAV. Its documentation describes control of all mail traffic with a focus on spam and virus detection; a web interface, message tracking, spam quarantine and high availability through clustering are part of the product. It works with existing mail servers such as Microsoft Exchange or Postfix. It can be installed from an ISO on a server of your own, on top of Debian, or as an LXC container on Proxmox VE.
Proxmox Server Solutions GmbH is headquartered in Vienna, Austria, and describes itself as an independent company whose only investors are its customers and supporters. Proxmox Mail Gateway is open-source software, with paid subscriptions for support and related services. Because you run the gateway yourself, you decide where mail traffic is processed, for example within the EU.