Every single email in every single inbox in the world is stored on a server. The incredible quantity of data requires huge server farms - gigantic centres with millions of computers which store and transmit information. These servers consume massive amounts of energy, 24 hours a day, and require countless litres of water, or air conditioning systems, for cooling. The more messages we send, receive and store, the more servers are needed - which means more energy consumed, and more carbon emissions.
Today, take the first step: clean your mailbox from old messages that you don't need anymore!
P.S. If you want to learn more about why this is important, here is an interesting article from the BBC.