An interesting feature of email is the ability to send one message from one location to many remote locations. It has allowed for a scalability that is reminiscent of a newspaper that was not familiar with traditional letter correspondence.
The creation of mailing lists has made sending to many easier. It also created a sense of shared space amongst the users in the list. This contained space can be public or private. That is, one might add themselves to the list or the owner of the list must add them. There are also systems such as Egroups, that bridge the gap between mailing lists and Usenet. This adds levels of exclusivity that begin to delineate boundaries within groups.
All of the interfaces make it evident that messages from different sources were brought over into the local user's space.