1. Go to Control Panel -> Server Administration -> Mail
2. Set the Outgoing SMTP Server.
3. Go to your page and add a Calendar portlet (the new version!) to your page.
4. Click into the Calendar's table, a few days before today, then click on Edit Details button.
5. Change the Start Date to today, and 15 minutes later to the present time. So it will be in the near future.
6. Set the Reminders to 10 Minutes.
7. Click on Save.
There will be an error message. See in the attachment.