You can sort notification recipients for specific monitors by explicitly selecting the target monitor in the notification group settings and linking it to a notification channel / notification group.

If specifying a check monitor, you can search for it using the xxxx portion of [plugin.checks.xxxx]configured in mackerel-agent.conf.

You can check the xxxx in 'Monitors' located at the top / bottom of the host's details screen.

You can also restrict other notification groups from being notified by enabling the "Notify this notification group only" option.

For more details, refer to Notification Groups - Mackerel Docs.