A tutorial is available at techrepublic.com on how to add an existing Gmail account to Windows Mail.
The article notes that Google has added IMAP capabilities to their their Gmail service so users can access their Gmail emails by using Windows Mail after configuring it.
There are a lot of steps to do this procedure and the author posted a step-by-step guide on how to do it. Screen shots of the Gmail and Windows Mail settings are included to make it easier for users to follow the procedure. The last step may take a few minutes depending on how many emails that a user has.
Source: Tech Republic

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Lee
I wonder how google plans on monetizing users who use external clients. Unless they are just writing off the money? After all, majority of their users probably still use the webmail interface.