How to send emails to an external adress by nagios notification tools?

[Nagios]

Hi,
I have successfully installed nagios on my RedHat 9. I want to notify some users about hosts problems by email (external like user at yahoo.com). So I have try yo use the notify_by_email command and I do not have anything in my emails!

Can you help me?
I join some of my configuration files.

Thanks in advance.
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