As you may already know, one of the new major feature of Exchange and SharePoint 2013 (which is also available on Office 365 with Exchange Online and SharePoint Online) is the Site Mailbox.
As reminder – in case of you don’t know anything about it, a site mailbox is a central email and document sharing account that is accessed from a SharePoint site. In some way, we could say this is the new (and better version) of SharePoint Incoming Email Document library 
As soon as a site mailbox is provisioned and you have been granted access to it, it appears within your Outlook profiles.
So, recently, I went into an issue where the site mailbox was being stuck in provisioning, after more than 72 hours waiting I was always getting the error:
Almost there!
We’re not quite done setting up your site mailbox. Please check back in a few minutes. If you see this message in half an hour, contact your helpdesk.
X-OWA-Error: Microsoft.Exchange.Clients.Owa2.Server.Core.OwaExplicitLogonException
X-OWA-Version: 15.0.959.25
X-FEServer:
X-BEServer:
Date:
Even after multiple tries to remove it from SharePoint (as well as from the Office 365 administration portal as a user account is being created for the site mailbox), I was still stuck with this error.
So the solution was simple:
- Open SharePoint Designer
- Connect to the SharePoint site where the site mailbox is stuck in provisioning
- Click on the Site Options button shown in the ribbon
- Locate all ExchangeTeamMailbox; there should be 4 entries
- Delete the entries, remove the site mailbox from the SharePoint site if still there and try to re create the site mailbox
Et voilà