FreePBX-16.0.21.9

mail config >> postfix smarthost to gmail 

postconf mail_version
mail_version = 2.10.1

mkdir /etc/postfix/sasl/
vim /etc/postfix/sasl/sasl_passwd
[smtp.gmail.com]:587 <your gmail user:pass>
chmod 600 /etc/postfix/sasl/sasl_passwd

ls -l /etc/postfix/sasl/

cp -a /etc/postfix/main.cf /etc/postfix/main.cf.bak

vim /etc/postfix/main.cf
(add)
relayhost = [smtp.gmail.com]:587
smtp_use_tls = yes
smtp_sasl_auth_enable = yes
smtp_sasl_security_options =
smtp_sasl_password_maps = hash:/etc/postfix/sasl/sasl_passwd
smtp_tls_CAfile = /etc/ssl/certs/ca-bundle.crt

postmap /etc/postfix/sasl/sasl_passwd

mynetworks = 127.0.0.0/8 

tail -f /var/log/maillog
or
postqueue -p

systemctl restart postfix.service

postqueue -f (flush)

mail -s "Test Subject" test@youremail