How do I open port 25 (or any other port)?
Allowing traffic to either enter or leave your Linode is done with
iptables. If you are unfamiliar with iptables then check out the following guide as it covers iptables in detail and will show you how to open or close specific ports:
The following iptables rule should help you get started:
sudo iptables -I INPUT -p tcp -m tcp --dport 25 -j ACCEPT
You can also use
ufw as an alternative to using iptables. UFW is covered here: