EXPROX.DLL fails after back-end ISAPI Extension WriteClient() call   

Hi,

I am stuck with an Exchange front-end/back-end problem where my ISAPI 
extension on the back-end server is trying to return an error message to the 
client browser.  The error message never gets there, and a few seconds after 
calling lpECB->WriteClient( ) from the back-end server, EXPROX.DLL fails on 
the front-end server with the following Event Log entry:

Source:         Application Error
Category:      (100)
Type:            Error
Event ID:      1000
User:            N/A
Description:   Faulting application w3wp.exe, version 6.0.3790.1830, 
faulting module exprox.dll, version 6.5.7638.1, fault address 0x0002ce94.

The back-end is configured to use Integrated Windows Authentication for the 
Exchange virtual directory.
The front-end is configured to use Basic Authentication, and will use SSL in 
the future, but currently is in the clear.

The code I am using in the back-end ISAPI extension is shown below, and 
works fine in a standalone Exchange configuration, or if I connect directly 
to the back-end server without going via the front-end server.  (Error 
handling has been removed for clarity)


void SendErrorResponse(const LPEXTENSION_CONTROL_BLOCK lpECB, const LPCSTR 
szStatus, const LPSTR szError)
{
 HSE_SEND_HEADER_EX_INFO HeaderExInfo;

 HeaderExInfo.pszStatus = szStatus;                                   // 
usually "426 Conflict"
 HeaderExInfo.pszHeader = "Content-type: text/plain\r\n\r\n";
 HeaderExInfo.cchStatus = static_cast<DWORD>(strlen( 
HeaderExInfo.pszStatus ));
 HeaderExInfo.cchHeader = static_cast<DWORD>(strlen( 
HeaderExInfo.pszHeader ));
 HeaderExInfo.fKeepConn = FALSE;

 BOOL bSuccess = lpECB->ServerSupportFunction( lpECB->ConnID, 
HSE_REQ_SEND_RESPONSE_HEADER_EX, &HeaderExInfo, NULL, NULL );

 DWORD dwBytesToWrite = static_cast<DWORD>(strlen( szError ));
 bSuccess = lpECB->WriteClient( lpECB->ConnID, szError, &dwBytesToWrite, 
HSE_IO_SYNC | HSE_IO_NODELAY);
}

Any thoughts would be greatly appreciated!

Thanks,
MikeC.

date: Tue, 23 May 2006 16:10:44 +0100   author:   Mike