Juliano, most of the things you ask are covered by the manual or are very basic things like the installation of Debian Stretch. I would suggest reading more documentation or attend to a linux-/mail-/administration-workshop.
apt-get install proxmox-mailgateway Reading package lists... Done W: There is no public key available for the following key IDs: EF0F382A1A7B6500 /etc/apt/sources.list #deb http://security.debian.org/ jessie/updates main #deb-src http://security.debian.org/ jessie/updates main #deb http://ftp.debian.org/debian/ jessie-updates main #deb-src http://ftp.debian.org/debian/ jessie-updates main #deb http://ftp.debian.org/debian/ jessie main #deb-src http://ftp.debian.org/debian/ jessie main deb http://ftp.debian.org/debian stretch main contrib deb http://security.debian.org stretch/updates main contrib deb http://download.proxmox.com/debian/pmg stretch pmg-no-subscription
