Today, I was troubleshooting a CentOS server for a customer experiencing issues with their Postfix mail server. The problem was that Gmail’s mail server was refusing the connection. After verifying that the Postfix configuration was correct, I was still unable to relay emails to Gmail. Upon further investigation, I discovered that the server was using CSF (ConfigServer Security & Firewall), which could be affecting the connection.
Here is a temporary fix…
In CSF make sure that those option a correctly setup
Allow gmail server IP in your ConfigServer Security & Firewall Allow List
As per today February 6th 2025, here is the official CIDR/IP list
64.233.160.0/19 # Google Gmail
66.102.0.0/20 # Google Gmail
66.249.80.0/20 # Google Gmail
72.14.192.0/18 # Google Gmail
74.125.0.0/16 # Google Gmail
108.177.8.0/21 # Google Gmail
108.177.96.0/19 # Google Gmail
172.217.0.0/19 # Google Gmail
173.194.0.0/16 # Google Gmail
207.126.144.0/20 # Google Gmail
209.85.128.0/17 # Google Gmail
216.58.192.0/19 # Google Gmail
216.239.32.0/19 # Google Gmail
108.177.16.0/24 # Google Gmail
108.177.17.0/24 # Google Gmail
142.250.220.0/24 # Google Gmail
142.250.221.0/24 # Google Gmail
Allow those IP and restart ConfigServer Security & Firewall
You should now be able to send and receive emails from a Gmail account. However, this is only a temporary solution until I resolve the IPv6 issue in ConfigServer Security & Firewall. If IPv6 is enabled, Gmail will prioritize it for email delivery. Make sure to open the necessary IPv6 ports used for sending and receiving emails to ensure proper functionality.
Ex.: TCP6_IN = 993,995 TCP6_OUT = 993,995