When I read an email, I want it to stay in my Inbox until I move or delete. I don't want you tossing it into "old mail". I'll manage myself.
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You can switch between New/Old Mail and Unified Inbox at will, but this must be done from a PC's browser. Here are the steps:
1. Click the Settings icon | select More Settings.
2. Click Viewing email.
3. Under Inbox style, select Unified Inbox.
4. Click Back to Inbox or Back to New Mail when done.