A rich hosted Exchange environment for every user without having to manage a server.
http://www.microsoft.com/exchange/en-us/exchange-online-hosted-email.aspx
Email Hosting Providers · Business Email Hosting
Get access to data on 6,959,402 websites that are Microsoft Exchange Online Customers. We know of 1,811,647 live websites using Microsoft Exchange Online and an additional 5,147,755 sites that used Microsoft Exchange Online historically and 1,081,723 websites in the United States.
Get a list of 6,959,402 websites using Microsoft Exchange Online which includes location information, hosting data, contact details, 1,811,647 currently live websites and an additional 2,092,481 domains that redirect to sites in this list. 5,147,755 sites that used this technology previously and 1,081,723 websites in the United States currently using Microsoft Exchange Online.