Smaller the business, higher the energy (KWh per user) spent on hosting the own email servers. These could go high as 175KWh or more. If the business decided to host the email with GMail (with their own domain name etc) this energy usage can drop drastically, to less than 2.2 KWh per user.
While large business with about 10,000 users could bring down the energy usage (7.6 KWh per user) it is still more than what GMail uses.
Google has published a paper, Google’s Green Computing:Efficiency at Scale (PDF), that explains the energy saving using the GMail cloud services.
Just to compare, if you are delivering videos over the sneaker net;
You’d have to watch YouTube for three straight days for our servers to consume the amount of energy required to manufacture, package and ship a single DVD.Our small company, with less than 10 users, used to host our own email server(s) but now all our email is is hosted with GMail. In addition to direct energy saving, we are saving much more energy spent on maintaining the servers and on top of it, energy and time spent on SPAM and Virus protection, DOS protection, and script kiddies handling among other things. The only servers that we run today are our DNS servers (two tiny little plugcomputer). Even those could go away, if Google can provide us with DNS services!
Gmail: It’s cooler in the cloud - Official Gmail Blog
also on Green Blog