This allows Proxmox (postfix) to use an external mail server:
Bash
apt install libsasl2-modules/etc/postfix/main.cf
# See /usr/share/postfix/main.cf.dist for a commented, more complete version
smtpd_banner = $myhostname ESMTP $mail_name (Debian/GNU)
biff = no
# appending .domain is the MUA's job.
append_dot_mydomain = no
# Uncomment the next line to generate "delayed mail" warnings
#delay_warning_time = 4h
alias_maps = hash:/etc/aliases
alias_database = hash:/etc/aliases
mydestination = $myhostname, localhost.$mydomain, localhost
relayhost = [smtp.gmail.com]:587
smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
smtp_sasl_security_options = noanonymous
smtp_sasl_mechanism_filter = plain, login
smtp_tls_security_level = encrypt
smtp_tls_CAfile = /etc/ssl/certs/ca-certificates.crt
mynetworks = 127.0.0.0/8
inet_interfaces = loopback-only
recipient_delimiter = +
compatibility_level = 2bash
chmod 600 /etc/postfix/sasl_passwd
postmap /etc/postfix/sasl_passwd
systemctl restart postfix