It turned out to be a massive undertaking, that I heavily underestimated, but I managed to compile Mail Gateway for ARM64. This makes it possible to install it on devices like a Raspberry Pi and many others:
ARM64 Packages: https://github.com/qemus/proxmox-mail-arm64
Docker container: https://github.com/dockur/proxmox-mail
ARM64 Packages: https://github.com/qemus/proxmox-mail-arm64
Docker container: https://github.com/dockur/proxmox-mail
