Alerts require Enhanced service - see alerts service plan summary. One gateway plus one sensor or weather station (minimum) must register to receive email or text (SMS) alerts. System Alerts are not user-configurable.
In the Readings section of your data, each data value has a label (e.g. Sensor). Select the Red + (above). Each configuration screen is self-documenting (see Temperature Alert):
When an alert is set, the configuration window and data screen respond. Alerts are sent (if service plan is active) in a similar 4-day cycle to system alerts:
1) immediate alert
2) 2 hours later
3) 1 alert a day for 3 days after
Your email filter must allow firstname.lastname@example.org and email@example.com to pass through.