In most cases it happens when site admin or Account admin activated some settings which redirect mail to unconfigured services.
go to page https://yourdomain.example/accounts/site_admin/settings#tab-features and ensure that following features are disabled:
- Use remote service for notifications
- International SMS
also you can check on account setting https://yourdomain.example/accounts/1/settings#tab-settings should be disabled:
- Suppress notifications from being created and sent out
