Hi and welcome,
that’s a recurring question here in the forum. So e.g. take a look in this old threads:
- Email Notification
- Email Notification Sent but Not Receving Email
- Not sending out notifications
- Icinga Notification - Not working
Short Summary:
Icinga delivers two bash scripts for notifications. You can use this or you use your own one. Icinga triggers the notification script. So would you have to do is to check if these scripts are running correctly on your systems (maybe something is mising e.g. installation, configuration etc.)