“send an email only if the alert is CRITICAL” .. that is what we do actually. What is defined as critical will display a 'red dot alert' in the web interface and send a mail or trap alert (if mail/trap alert is ON for that counter).
Please confirm that you understand how to change the 'critical alert level' for a counter and how to turn off/on email alerts for counters. If you are not familiar with this I think we should start here.