Proxmox Mail Gateway 5.1 We are pleased to announce the availability of Proxmox Mail Gateway 5.1. The Mail Gateway is based on Debian Stretch 9.5 with a 4.15 kernel, and the new version 5.1 comes with Debian security updates, bug fixes, and GUI improvements. The new Transport Layer Security (TLS) policy provides certificate-based authentication and encrypted sessions; the user management now allows a  help desk role; editing and showing smarthost port is possible; and we included support for SMTPUTF8

REF: https://forum.proxmox.com/threads/proxmox-mail-gateway-5-1-available.47798/