Step 3. Setting up Proxmox Mail Gateway.
Proxmox Mail Gateway or PMG is a shell that provides a neat interface for Sendmail / Postfix. It adds a number of features but also adds quite a bit of complexity. It will be rare for you to find a PMG specific article describing how to fix a Sendmail configuration issue or how to setup a PMG system from scratch never-mind getting it to work properly and then monitoring the system.