Is there any way to read via Microsoft.Office.Interop free space left in default Mailbox ?
(So the same info what is displayed in Outlook's down left corner - as shown below)
You can try to read the following properties using Store.PropertyAccessor.GetProperty():
PR_QUOTA_RECEIVE (DASL name http://schemas.microsoft.com/mapi/proptag/0x341C0003)PR_PROHIBIT_RECEIVE_QUOTA (DASL name http://schemas.microsoft.com/mapi/proptag/0x666A0003)PR_QUOTA_SEND (DASL name http://schemas.microsoft.com/mapi/proptag/0x341B0003)PR_PROHIBIT_SEND_QUOTA (DASL name http://schemas.microsoft.com/mapi/proptag/0x666E0003)PR_QUOTA_WARNING (DASL name http://schemas.microsoft.com/mapi/proptag/0x341A0003)PR_STORAGE_QUOTA_LIMIT (DASL name : http://schemas.microsoft.com/mapi/proptag/0x3FF50003)PR_MESSAGE_SIZE_EXTENDED (DASL name http://schemas.microsoft.com/mapi/proptag/0x0E080014)