As a first step we would suggest going through the CMS Post installation guide as this explains exactly what needs to be done to get email notifications working properly: https://xibo.org.uk/docs/setup/xibo-cms-post-installation-setup-guide and use the latest stable 2.3.6 release.
Updates have been made, now to 2.3.7
And I checked once again the CMS Post installation guide. As far I can see, everything is done.
Concerning the notifications, nothing changed: for an offline-display I get one, if this display returns, I get none.
I forgot to mentions: the server runs PHP 7.3.22