/etc/postfix/main.cf, with TLS authentication on port 587.
relayhost = mysmtp.domain.com:587 smtp_use_tls = yes smtp_tls_security_level = encrypt smtp_sasl_auth_enable = yes smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd smtp_sasl_security_options = noanonymous sender_canonical_maps = static:email@example.com
relayhost is the SMTP server you want to use. It must appear exactly as it is in
/etc/postfix/sasl_passwd (all the ] and [ must be the same).
smtp_tls_security_level must be
encrypt for port 587 and
may for port 25.
sender_canonical_maps might not be respected by the SMTP server, changed on the fly or even bounced discarded. This depends on the provider.
main.cf, do not forget to:
service postfix restart