[Mailman-Users] What's the /var/spool/mailman/virgin for?

I've got an instance of mailman where the contents of /var/spool/mailman/virgin holds 1.4G of data. I'm just curious what this directory is for to determine what is putting the data there and what's responsible for sending it along. The same directory is empty for most other mailman instances I have access to.

Thanks in advance,

--Tony