[root@ns1 ~]# telnet gmail-smtp-in.l.google.com 25
telnet: connect to address 220.127.116.11: Connection timed out
telnet: connect to address 2607:f8b0:400d:c0f::1b: Network is unreachable
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It’s possible you you need to allow outgoing TCP traffic on port 25
iptables -I INPUT -p tcp -m tcp --dport 25 -j ACCEPT
Additionally, you can further read this DigitalOcean article explaining more about Iptables and what they are - https://www.digitalocean.com/community/tutorials/iptables-essentials-common-firewall-rules-and-commands
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