Setting-up Nagios Mail Notification Alert on Ubuntu 16.04 LTS

I have nagios core installed on Ubuntu Server 16.04. I have to send a mail alert to when services hit their warning/critical threshold. I have defined in contacts.cfg. Other configs seem to be aligned too. I can’t seem to find out how to send mail alert via nagios.
I installed mailutils by sudo apt-get install mailutils. Step by step tutorial would be great! Thanks a lot.

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