Your VPS server or scripts are configured to send email using GMail's SMTP but emails are not sent or you get an SMTP Error when trying to configure.
Solution:
You must allow these IP addresses on your csf (ConfigServer Security & Firewall) on WHM:
74.125.0.0/16
You have to allow outgoing traffic to ports 25, 465, 587. You should actually be fine enabling only the one you will connect to. So:
sudo vim /etc/csf/csf.conf
add 25,465,587 to TCP_OUT, and to TCP6_OUT if you use IPv6.
You might also need to remove that ports from SMTP_PORTS if you have SMTP_BLOCK enabled (if it is set to 1).
