Sometime back I posted an article about how to "Give Calendar Read Permission on all Mailboxes" in your environment and anyone can open anybody's calendar in Outlook easily if it is shared but how can anyone open in OWA? Let's discuss the caveats in detail...

Let's take an example, Sheldon Labrooy shared his calendar with me (Amit Tank) and I got a sharing invitation.

I can open his calendar by clicking on "Open this Calendar" button on top of the mail or with help of "Open a Shared Calendar".

But there isn't any option to open Sheldon Labrooy's calendar in OWA.

If I try to open his mailbox in "Open Other Mailbox",...

I got an error that you don't have permission to open this mailbox.

Pretty clear, he has shared only Calendar folder of his mailbox but not Full Mailbox.

When we try to open somebody else's mailbox in OWA, OWA main web page has all modules (Mailbox, Calendar, Tasks, Notes, other options etc) listed and Exchange tries to check full mailbox permission which we don't have and get permission error.

Hummm, to open other's mailbox in OWA we need to have explicit "Full Mailbox" permission on that mailbox, very well explained in below article.

How to Open Another User's Calendar in Outlook Web Access

When user or admin shares a calendar, it gives "Reviewer" MAPI Folder permission on just Calendar folder.

So how do we open shared mailbox in OWA, what's the hack?

Instead of opening main page of OWA why don't we directly open just calendar module part, and here are the links to open just calendar in various views...