I had made something like this before for refrigeration rooms.
But some info is missing about your project:
Send email, how to connect to email server? Wire network, WiFi, or GSM modem?
Power to the thing? 110 Volt / 230 Volt / 12 Volt / 24 Volt? AC or DC?
There are some pitfalls in measuring temperature in larger rooms depending on the construction of the defrosting of the evaporator, so info regarding this system must be present to escape/eliminate this issue.
What I can offer is a description on what gear/hw you need, how to connect all these together, it will be based on Raspberry Pi, DS18S20 temperature sensor, some display for Raspberry Pi to show settings (temperature limits) and some push bottoms to operate it.
Be aware that in some cases an electronic construction may not be able to operate in some environment for different reasons, in short: If you can't figure out your self how to install electronic components in an EMC and electrical safe way you may be better off by using industrial grade ready made modules like a "temperature measuring device" and a "GSM email message device".