In Opera 12 or earlier:
Assign desired shortcut to: Read mail, 2
That is the Received folder, where all read and unread mail is found. This is my entire Mail Access Points Menu from Opera 11.64.[Mail Access Points Menu]
Item, "Unread" = Read mail, 1,,,"Mark as read"
Item, "Received" = Read mail, 2,,,"View from"
Item, "Outbox" = Read mail, 3,,,"Send Mail"
Item, "Drafts" = Read mail, 4,,,"Compose Mail"
Item, "Sent" = Read mail, 5,,,"View to"
---Trash---
Item, "Spam" = Read mail, 8,,,"Mark as spam"
Item, "Trash" = Read mail, 6,,,"Show Trash"
---Feeds---
Item, "Mailing Lists" = Read mail, 9,,,"Show Mailing Lists"
Item, "Newsfeeds" = Read mail, 11,,,"Show Newsfeeds"
Huh? What? What are you talking about? What do I do to make a shortcut within Opera Mail? Are you telling me to edit some file by hand or something? I have no idea what you're talking about.