Proxmox Mail Gateway is a complete operating system based on Debian Buster 10.9, but using Linux kernel 5.4, which is under long term support (LTS) status. It protects organizations against threats against threats, such as spam, viruses, Trojans, and phishing emails. It is now possible to upload custom certificates from the web interface, or set up a cluster-wide ACME account to automatically get and renew certificates from an ACME provider. The certificate file handling is implemented in the Rust programming language.
Source: https://www.helpnetsecurity.com/2021/03/31/proxmox-mail-gateway-6-4/