Gmail stores all email with a label
Some labels are NOT visible to the user.
Emails in your inbox are assigned the label "Inbox".
This label becomes visible when you click on "All Mails".
All read messages are assigned label "read"
Unread messages are stored with the label "unread".
To view all the unread messages, in the search box enter:
label:unread
For read messages, enter:
label:read
~ via How to view all unread mails in Gmail - Instant Fundas