You can send a negative acknowledgment by posting a WM_DDE_ACK message with the DDE_NOTPROCESSED flag set in the status word of the shared-memory data structure. By not specifying DDE_FACK, it is legal to specify DDE_NOTPROCESSED, but only if the message is not supported, such as WM_DDE_POKE for the specified item. DDE_NOTPROCESSED is not the negative respond. The following code fragment shows how to do so:

/*  Specify the status flag, when allocating shared memory */
pDDEdata->fstatus &= 0;
    .
    .
    .

/*  Post the message */
WinDdePostMsg(hwndDest,           /* Handle of destination  */
              hwndSource,         /* Handle of source       */
              WM_DDE_ACK,         /* Message                */
              pDDEdata,           /* Shared-memory pointer  */
              DDEPM_RETRY);       /* Retry                  */

If an application is busy when it receives a DDE message, it can post a WM_DDE_ACK message with the DDE_FBUSY flag set.