2147746063 is 0x8004010F, which is MAPI_E_NOT_FOUND error.
How do you retrieve the property? If the property cannot be retrieved,
MAPI_W_ERRORS_RETURNED is returned by GetProps() and the returned property
is reset to PT_ERROR. SPropValue.Value.err is then set to the appropriate
AFAIK that property is calculated when the attachment and message are saved,
they are not available before that.
Dmitry Streblechenko (MVP)
OutlookSpy - Outlook, CDO
and MAPI Developer Tool