When running Exchange 2007 in a testing lab with limited resources you might experience issues in mail delivery like emails goes to draft folder or not receiving them at all. The reason for that is the Back Pressure feature that stops specific functions.
To disable Back Pressure: Go to C:\Program Files\Microsoft\Exchange Server\Bin directory and open the EdgeTransport.exe.config file using notepad.
Change the value of EnableResourceMonitoring from True to False: To: Save file and Restart Microsoft Exchange Transport Service. –