I was having problem today to send out an email from my Fedora 11 Linux box. By default, the mail server software, Sendmail, comes with the standard installation and no configuration is required. I didn’t understand why the Sendmail didn’t work.

After a very detail investigation, I finally found out why. Here are the steps I went through to solve the problem.

1. Make sure the Sendmail is running.

netstats -na | grep LISTEN

You should see something is running on port 25.

2. Make sure the stupid SELinux Firewall is disabled.

3. Make sure the Linux Firewall allows port 25.

4. Try to connect to the port 25.

telnet localhost 25

If you have trouble to connect to the port, that means something is not work.

5. Make sure you are not blocking port 25 at the system level:
In /etc/hosts.allow

sendmail: localhost: allow

That should work!

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