Need proof that distribution groups copy email to mailboxes...

My supervisor doesn't believe me when I say that distribution groups work like this:

Mail is received, copies of mail are sent to members of distribution group. Distribution group does not retain the message anywhere since it's not a mailbox.

I suppose I could be wrong, but fundamentally I was under the impression that's how they work. That's how I learned it anyway. Does anyone have a link or anything to something official that explains this so I can move on?

This is a silly request, I know.