Configuring User E-mail Address
Simple Configuration via Web UI
By default, Proxmox will try to use the domain portion of the "Email from address" as the e-mail relay server.
Advanced Configuration via CLI
To make more advanced configuration changes, like using a gmail account, you need to edit the postfix settings via command line
Sources: https://forum.proxmox.com/threads/get-postfix-to-send-notifications-email-externally.59940/,
https://www.reddit.com/r/homelab/comments/5nzmm3/setting_up_the_proxmox_email_alerts/