Hi,
Open System Manager, Expand Global settings, Click on Internet Message Formats, Right click on "Default" in the right hand pane and choose properties, choose the "Advanced" tab, Make sure the option "Allow Non Delivery Reports" is unchecked.
Bad mail goes to the Bad Mail folder. What is bad mail? It usually relates to a message that was undeliverable and can't get a NDR back to the originating system. and has three parts to it: (0083.bad, 0083.bdr, and 0083.bdp (I left out a lot of the Hex for brevity)).
By turning off the Allow NDR, your system will only report internal NDR's to this folder. This will also reduce how fast that folder fills up. I would also assume you have relaying turned off and that you have checked for possible internal spammer(this is a stretch, but it has been known to happen).
Take what you need, disregard the rest, and hopefully I have helped you a little.
Wyz