To be able to send emails from a linux server via a smarthost just add this to your /etc/postfix/main.cf:
smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = static:smtp@example.com:mypassword
smtp_sasl_security_options = noanonymous
smtp_tls_security_level = encrypt
relayhost = mail.example.com:587I use this on my home-server/NAS behind a DSL line to get the root-messages (refer https://www.bergercity.de/general/postfix-redirect-mails-for-root/).