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Taking Gmail as an example, put the following into
/etc/postfix/main.cf
.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 smtp_tls_CAfile = /etc/pki/tls/certs/ca-bundle.crt smtp_use_tls = yes
smtp_tls_security_level = encrypt