When creating a new Recipe, drag&drop Mobile Alerts temperature sensor and Mail actuator on the rule sheet and connect them.
By clicking on the Mobile Alert sensor, you shall set up the upper and the lower bound. Select e.g. 5°C as lower bound and also insert an upper bound (in this case the exact value is irrelevant, it can be 20°C).
By clicking on the Email actuator, select "Below" from the drop-down menu of the "Trigger on states" section and also delete the "In Range" tab.
From now on, you will be notified if the temperature is sinking below 5°C.