setup gmail relay

cf: https://www.howtoforge.com/tutorial/configure-postfix-to-use-gmail-as-a-mail-relay/#debian-ubuntu

1. edit /etc/postfix/sasl_passwd, add this line
   [smtp.gmail.com]:587 username@gmail.com:password 
2. chmod 0400 /etc/postfix/sasl_passwd, 
3. vi /etc/postfix/main.cf, add these lines:
   #relayhost = (uncomment this line)
   # chiao
   relayhost = [smtp.gmail.com]:587
   smtp_sasl_auth_enable = yes
   smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
   smtp_sasl_security_options =
   smtp_tls_CAfile = /etc/ssl/certs/ca-certificates.crt
   smtp_use_tls = yes 
4. postmap /etc/postfix/sasl_passwd 
5. service postfix restart