When you call the IAttach::SaveChanges(KEEP_OPEN_READONLY) function in your MAPI application, Extended MAPI may unexpectedly return a MAPI_E_NO_SUPPORT value

...

HRESULT hrRetVal = S_OK;

...

// Open attachment - for simplicity we're assuming to have valid data in pMessage and dwAttachNum

hrRetVal = pMessage->OpenAttach(dwAttachNum, NULL, MAPI_MODIFY, &pAttach);
if (hrRetVal == S_OK)
{
                // Modify attachment
                ...
                
                // Commit attachment changes 
                hrRetVal = pAttach->SaveChanges(KEEP_OPEN_READONLY);
                if (hrRetVal != S_OK)
                {
                                // When the original message has been generated using OWA 2000 or OWA 2003 
                                // we may be facing an unxpected return code of MAPI_E_NO_SUPPORT here
                                ...

                }               // if
 
                // Release attachment object

                pAttach->Release();

}               // if

...