Long time ago, I had to retrieve the mailboxes configured in Outlook, allowing to select the right one.
It was not so easy to find the answer, so here it is
Code:
Public Sub Outlook_DisplayAllMailbox()
   ' #VBIDEUtils#***********************************************************
   ' * Author           :
   ' * Web Site         :
   ' * E-Mail           :
   ' * Date             : 01/24/2007
   ' * Time             : 13:42
   ' * Module Name      : Outlook_Module
   ' * Module Filename  : Outlook.bas
   ' * Procedure Name   : Outlook_DisplayAllMailbox
   ' * Purpose          :
   ' * Parameters       :
   ' *                    oSelect As Control
   ' * Purpose          :
   ' **********************************************************************
   ' * Comments         :
   ' *
   ' *
   ' * Example          :
   ' *
   ' * See Also         :
   ' *
   ' * History          :
   ' *
   ' *
   ' **********************************************************************

   ' #VBIDEUtilsERROR#
   On Error GoTo ERROR_Outlook_DisplayAllMailbox

   Dim objOutlook       As Object

   Dim nI               As Long

   Set objOutlook = CreateObject("Outlook.Application")

   On Error Resume Next
   For nI = 1 To objOutlook.session.Accounts.Count
      Debug.Print objOutlook.session.Accounts.Item(nI)
   Next
   On Error GoTo ERROR_Outlook_DisplayAllMailbox

EXIT_Outlook_DisplayAllMailbox:
   On Error Resume Next

   Set objOutlook = Nothing

   Exit Sub

   ' #VBIDEUtilsERROR#
ERROR_Outlook_DisplayAllMailbox:
   If Err.Number = -2147467259 Then
      MsgBox Error, vbInformation
      Resume Next
   End If

   Resume EXIT_Outlook_DisplayAllMailbox
   Resume

End Sub