Understanding Recipient Types

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It is tempting to think of a recipient as a mailbox or simply as an object that can receive a message, and as you administer your organization, it may be convenient to take that view. But it is important to understand the ways in which the underlying architecture affects how you work with recipients in Exchange Server.

In Exchange Server, a recipient object does not receive messages. Instead, it is a reference to a resource that can receive messages. This is a subtle but important distinction. Recipient objects are contained in and maintained by Active Directory. The resources that those objects reference could be anywhere. One resource might be a mailbox for a user in your organization. A mailbox resource would be contained in the mailbox store of a particular Exchange server and maintained by its Information Store service. Another resource might be a user on the Internet. In this case, the recipient object would contain a reference to that resource, along with rules governing the transfer of messages. Four types of recipient objects are available in Exchange:

• User A user in Windows 2000 is any individual with logon privileges on the network. With regard to Exchange Server, each user in Active Directory can be mailbox enabled, mail enabled, or neither. A mailbox-enabled user has an associated mailbox on an Exchange server. Each user mailbox is a private storage area that allows an individual user to send, receive, and store messages. A mail-enabled user is one that has an email address and can receive, but not send, messages.
• Contact A contact is essentially a pointer to a mailbox in an external messaging system, most likely used by a person outside the organization. This type of recipient points both to an address to be used to deliver messages sent to that person and to the properties that govern how those messages are delivered. Contacts are most often used for connecting your organization to foreign messaging systems, such as Microsoft Mail, Lotus cc:Mail, or the Internet. An administrator creates contacts so that often-used e-mail addresses are available in the Global Address List as real names. This makes it easier to send mail because users do not need to guess at cryptic e-mail addresses.
• Group A group in Windows 2000 is an object to which you can assign certain permissions and rights. Users who are placed in a group are automatically given the permissions and rights of the group. Exchange 2000 Server uses the concept of mail-enabled groups to form distribution lists. Messages sent to a group are redirected and sent to each member of that group. Groups can contain any combination of the other types of recipients, including other groups. These groups allow users to send messages to multiple recipients without having to address each recipient individually. A typical group is the one named Everyone. All Exchange recipients are made members of the Everyone group. When a public announcement is made, the sender of the announcement simply selects the Everyone group and is not forced to select every user's mailbox from the Global Address List.
• Public folder A public folder is a public storage area, typically open to all users in an organization. Users can post new messages or reply to existing messages in a public folder, creating an ongoing forum for discussion of topics. Public folders can also be used to store and provide access to just about any type of document. The concept of a public folder as a recipient is difficult to grasp because the repository for information is shared. One way that a public folder is used as a recipient is when it is configured for a Network News Transfer Protocol (NNTP) news feed. Under this arrangement, the information from the newsgroup is sent to the public folder recipient and can then be viewed by Exchange users in the organization.

Although a public folder is a type of recipient, it performs many more functions than just transferring or receiving messages. For that reason, this chapter focuses on the other recipient types: users, contacts, and groups. Chapter 10 is devoted to a full review of the features, functions, and administration requirements of public folders.