Email notifications from OEM

I am trying to generate email alerts from OEM to multiple email addresses. I have configured the SMTP server details from 'Setup' screen. And then I have added the email addresses under the 'Preferences' tab in OEM. To check if the alerts are working fine, I have shutdown database and restarted 3/4 times. But only once the alerts notification came and that too only to one email address and not to all the email ids mentioned. Please let me know why this is working in this way. Also, am I missing anything?? Do I need to add anything else to get the notification emails correctly for all the email ids?? Also, there is one screen for notification schedule. Do I have to define some schedule???