In Hotmail you have the facility to connect other mail accounts of yours. So in a single log on you can check other inboxes as well. This is a cool feature as we can link non Hotmail accounts as well. I have linked my Gmail with my Hotmail.
When you do this you have to provide the credentials of the Gmail account and connect it to your Hotmail. Actually Hotmail does not open the Gmail inbox inside your Hotmail, but it asynchronously downloads the mails from the Gmail. (this is bit not nice, but there might be policy boundaries among the companies).
So the bad side is, if I open and read a mail sent to my Gmail, from the Hotmail; and when I login to the Gmail it still shows that particular mail as unread. So my credentials are used to connect to the Gmail, and not for maintaining any sync, it is just for the download.
And we think this is right. Because what ever happens there might be organizational policies that prevent to do this. Simply Microsoft does not have permissions to edit Google inboxes. 😛
Another scenario is, since this connection is a mere download of the mails there is no impersonation is implemented. I’ll explain my real experience here, I have signed to a Google group using my Gmail ID. So any mails sent by them obviously come to my Gmail account as well as my Gmail folder in the Hotmail.
When I clicked the mail from the Hotmail and it had a link to a shared Google doc among the group members, When I clicked on the link, I was prompted to sign in to my Google account. That’s the point, there is no impersonation. (or my Hotmail has no way to explicitly pass the information that I’m the user of the same Google mail, though it has my Google mail credentials).
They use the credentials only for downloading the mails and not for any explicit impersonations. :D.