External email to distribution list

Is there any way to enable external people to send to an Exchange distribution list? I would like that kind of listserv functionality in Exchange. In other words, I’d like user@overthere.com to be able to send to list@mydomain.com. It works fine theother way: I include external users on my distribution lists (as Custom Recipients) and they receive all internal mail sent to the List. But they can’t initiate mail to the list.