По аналогии с GMAIL, только другой хост.
В main.cf:
relayhost = [smtp.gmail.com]:587
# use tls
smtp_use_tls=yes
# use sasl when authenticating to foreign SMTP servers
smtp_sasl_auth_enable = yes
# path to password map file
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
# list of CAs to trust when verifying server certificate
smtp_tls_CAfile = /etc/ssl/certs/ca-certificates.crt
# eliminates default security options which are imcompatible with gmail
smtp_sasl_security_options =
Add Gmail creds to /etc/postfix/sasl_passwd
[smtp.gmail.com]:587 username:password
Run
sudo postmap /etc/postfix/sasl_passwd