I needed to configure Proxmox Mail Gateway to use an authenticated SMTP relay for outgoing mail. There is no way to add a username and password in the PMG GUI; however, you can do it in the command line and it follows standard postfix syntax. Thanks to this post for helping me get it set up: https://forum.proxmox.com/threads/relay-username-and-password.129586

To get it to work you have to drop to the CLI and configure your username and password. Then copy the template file over and make your changes there, as editing postfix directly gets overwritten with subsequent GUI changes.

mkdir /etc/pmg/templates/

cp /var/lib/pmg/templates/main.cf.in /etc/pmg/templates/main.cf.in

create /etc/postfix/smtp_auth and populate it:

relay.host.tld   username:password

Append the following to the /etc/pmg/templates/main.cf.in template:

smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = hash:/etc/postfix/smtp_auth
smtp_sasl_security_options = noanonymous

Change permissions to smtp_auth file, run postmap to generate the db with the password, and run pmgconfig to refresh the configuration.

chmod 640 /etc/postfix/smtp_auth
postmap /etc/postfix/smtp_auth
pmgconfig sync --restart 1

This worked for me.