Abstract: Subscribers (100-108) of an electronic mail system (1-2) service are divided into a plurality of subscriber communities (30-34). Subscribers in each community have common rights of access to one or more communication service--the sending and receiving of mail messages--which rights are generally different from those of subscribers in other communities. Permissions to send and receive messages between any two communities are specified on each mail system in a permissions matrix (40), wherein each row (41) represents a different sender community, each column (42) represents a different recipient community, and the value stored at their intersection (43) indicates the permission vis-a-vis the two communities. A function (50) is invoked on the sender's mail system (1,2) at message addressing, and on the sender's and recipients' mail systems (1,2) at message delivery, to enable or prevent (FIG. 3) message sending or delivery according to the matrix-specified permissions.