Pretty limited, because http://mail.google.com/mail/feed/atom/inbox/ AND https://mail.google.com/mail/feed/atom/ shows all unread mail, not just a folder " Inbox "as it should be in my version. Then we could have /read and /unread , but instead /read does not exist, and /unread gives you all unread, not just unread from the inbox.
So http://mail.google.com/mail/feed/atom/all may be what you want as it gives you literally ALL mail. At the moment, using the atomic channel alone, you could receive ALL mail as you requested, but I'm not sure what you are asking ... I would suggest that you want ALL (which means reading / unread) from INBOX. At the moment, this is not possible using the feed, and you are stuck with all the mail from all the shortcuts where they are required and probably not all live in the Inbox (many of them have the ability to skip mailboxes purposefully).
Literature:
http://googlesystem.blogspot.com/2008/03/feed-for-unread-gmail-messages.html
https://developers.google.com/google-apps/gmail/gmail_inbox_feed
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