Hello,
The Badmail folder in SMTP5 is going to act the same as the similar folder in IIS. This is for mail that for some reason was undeliverable by the MTA the folder resides in. You should generally see mail in the delivery queue badmail folder. (If any)
Most often these are NDR messages that we not able to deliver back to an incorrect address. So the fact that you are not seeing much mail in here is a good thing. If you view the messages in a txt viewer you can determine if they are ok to be removed, or as stated before, they were unable to be delivered and as such are likely ok to remove anyways. This, however, would be up to you.
Hope this helps!
Tom