Hotmail, the web email service from Microsoft, has introduced a really cool new feature. Now you will be able to send and receive emails with any address, including those from Gmail and Yahoo! — keep in mind that in order to include a Yahoo! email address, you need the POP service that you can only get with Yahoo! Mail Plus –. But do not confuse this new feature; Hotmail has had the send and receive email via POP service for a long time, and the same happens with Outlook with the Send As option. These features did not change. What is interesting about this new feature is that, if you don’t have an email account with Hotmail for whatever the reason, you don’t have to create a new address just to try Hotmail; you can still use Hotmail web email service as your email client with another address from a different service, creating a Windows Live ID.
To be able to send and receive emails using Hotmail, you need to create a Windows Live ID here, with any email address from any email service that you already have. After you verify your email address, you can then get access to Hotmail with the address that you used, e.g. “firstname.lastname@example.org”.
Source Inside Windows Live