Once the notification method is triggered by bpnotify_nxd runs a .exe or .bat, that process keeps running and never finishes.
This causes performance issues when several processes run.
SDM 17.x
SDM does not have the capability to end the process
The executable will run on its own, all SDM does is invoke the executable. SDM isn't going to perform garbage collection to kill it.
You would need to run a script through Windows Scheduler, that after a time, locate its PID and kill it (ie : "taskkill /IM <process-name> /F ").
Another option is to add to the .exe to close the process once the email is sent and all the actions are completed.