# Specify zero or more "type:name" lookup tables. Type is a service defined in master.cf
cloudservice@example.org    [smtp.gmail.com]:587

cd /etc/postfix && postmap relay_by_sender

/etc/postfix/sasl_passwd

[smtp.gmail.com]:587  myrealsender@gmail.com:secret_password

cd /etc/postfix && postmap sasl_passwd
sudo chown root:root /etc/postfix/sasl_passwd /etc/postfix/sasl_passwd.db
sudo chmod 0600 /etc/postfix/sasl_passwd /etc/postfix/sasl_passwd.db

main.cf

# By default this is standalone mail server, so relayhost= is empty
relayhost =

# A sender-dependent override for the global relayhost parameter setting
sender_dependent_relayhost_maps = hash:/etc/postfix/relay_by_sender

service postfix restart

Test email: cat /etc/issue | mailx -r “cloudservice@example.org” -s “test relaya” “destination@email.org”

(SASL authentication failed; cannot authenticate to server smtp.server.org[11.22.33.44]: no mechanism available)