I'm trying to set up mailbox for fetchmail. But the other way around as offered by PGM: Proxmox GM shall deliver accepted e-mails to a local mailbox from several (relay?) domains onto the PGM host. This mailbox will be fetchmailed by another host (from behind a firewall) which then actually delivers the e-mails to the final e-mail servers in a secured network.
Long story short: PGM can't push e-mails through the firewall, but we can pull e-mails from behind.
My thought was to set up a Mail Proxy, but I don't know what to enter into the Host field...