Private Email Networks (PENs) are very safe. The Internet address (URL) is indeed public, but this web address isn't useful unless a person belongs to a particular Private Email Network (PEN).
Using the administrative controls provided by email2, PEN Owners are able to restrict access and create "Controlled Access" PENs which do not allow users to join unless they are explicitly invited by an administrator. Any users that are no longer needed in the Private Email Network, or that are abusing it, can be quickly removed by the administrator.
Even when using a relatively open Private Email Network, all users must register and confirm their identities before using the Private Email Network for communicating with their friends and associates.
// Edited July 2008