Don't pass MAPI_UNICODE as this will cause it to fail. You can pass NULL. Here is a small example for you and you can fix the rest of it but you get the idea.

// Declare a NULL SRowSet structure, and populate it with property values from one of the message stores by using IMAPITable::QueryRows. The following code retrieves the first (default) message store:

if (SUCCEEDED(hr = pSession->GetMsgStoresTable(0, &pTable)))
{

   //Declare a NULL IMsgStore interface object, and open the message store by using IMAPISession::OpenMsgStore - making use of the     SRowSet structure:
   if (SUCCEEDED(hr = pTable->QueryRows(1, 0, &pSRowSet)))
   {    
        if(FAILED(hr = pSession->OpenMsgStore(0, pSRowSet->aRow[0].lpProps[0].Value.bin.cb,
              (ENTRYID *)pSRowSet->aRow[0].lpProps[0].Value.bin.lpb, NULL, 0, &pStore)))
       {
           print("OpenMsgStore");
           return hr;
       {

   }
   else
   {
       print("QueryRowserror");
       return hr;
   }

}
else
{
   print("GetMsgStoresTable error");
   return hr;
}

// If no longer needed, release the message stores table by calling IUnknown::Release on the table object, and free the memory allocated for the row set structure by calling FreeProws:
pTable->Release();
FreeProws(pSRowSet);

 Hi,
 I was trying to get mailbox store information using
 "GetMsgStoresTable" MAPI call.

 hr = pSession->GetMsgStoresTable(MAPI_UNICODE, &pTable);

 But its failing as MAPI_E_BAD_CHARWIDTH. In net I found that this is
 something to do with UNICODE ( The MAPI_UNICODE flag was set and MAPI
 does not support Unicode at present.) but not getting how to fix this
 so that I can get these store information. If any one is having sample
 code or hint on proceeding further on this please let me know.
 Regards,
 Shivaraj