Exchange is a "one instance" email server so there's only one copy of any given message in the PRIV.EDB on the local Exchange server.
If you leave a message for a big DL, everyone gets a "pointer" to that message and the size of the message does count against your quota, but the message is only copied once. If there is more than one Exchange server in the site, each server gets a copy of the message for its PRIV.EDB file, as such when you connect to your Exchange server via Outlook, you only have to drag the attachment from your server to your client, not all the way from the originating home server.
Jeff Lindborg
Unity Product Architect
Active Voice Corp
jlindborg@activevoice.com