Modern communication and computer networks provide users with ready access to rich sources of information. For example, many vendors provide online product information, discount coupons, purchasing information, etc. Accessing such information often requires no more effort than navigating to the vendor's web site, or otherwise using an application, web browser, or other “user agent” to communicate with a vendor's server.
However, vendors and other information providers frequently require users to leave email addresses, postal addresses, telephone numbers, or other personally identifying information. Some companies use identifying information only to provide the information in which users have expressed an interest, and they do not make such information available to third parties. Not all companies exercise such restraint. For example, personally identifying contact information provided by a user to one company frequently ends up being shared or sold to other companies, one or more of which may use the information to spam the user, or to otherwise send unsolicited information of questionable interest to the user.