If you want to understand more about how sending fewer emails saves energy take a look at these articles:
Building a greener internet: How can Europe play a leading role in building a more sustainable internet? https://www.nesta.org.uk/blog/building-greener-internet/
Dark satanic mills 2.0: inside the massive server farms storing your data—and harming the planet https://www.prospectmagazine.co.uk/other/dark-satanic-mills-2-0-inside-the-massive-server-farms-which-store-your-data-but-harm-the-planet