The same way how exchange server refer to.
For an exchange user, when a mail(which contains embeded image in body)
receive on exchange server, i open this mail using CDOEX IMessage and
IDataSource interfaces and create eml file using IMessage->Stream->SaveToFile.
IMessagePtr pMsg( __uuidof( Message ) );
IDataSourcePtr pDsrc = NULL;
USES_CONVERSION;
HRESULT hr = pMsg->get_DataSource(&pDsrc);
hr = pDsrc->Open( "mailpath",
NULL,
adModeReadWrite,
adFailIfNotExists,
adOpenSource,
_bstr_t() , _bstr_t() );
pMsg->GetStream()->SaveToFile("c:/emlfilename.eml",adSaveCreateOverWrite);