The people and resources that send and receive messages are 
the core of any messaging and collaboration system. In an 
Exchange Server organization, these people and resources 
are referred to as recipients. 

A recipient is any mail-enabled object in the Active 
Directory directory service to which Exchange can deliver 
or route messages. This topic discusses the recipient types 
that are supported in Microsoft Exchange Server 2007.

User mailbox	

A mailbox that is assigned to an individual user in your 
Exchange organization. It typically contains messages, 
calendar items, contacts, tasks, documents, and other 
important business data.

Linked mailbox

	A mailbox that is assigned to an individual user in 
a separate, trusted forest.
 
Shared mailbox	

A mailbox that is not primarily associated with a single 
user and is generally configured to allow logon access for 
multiple users. 

Legacy mailbox	

A mailbox that resides on a server running Exchange Server 
2003 or Exchange 2000 Server. 

Room mailbox	

A resource mailbox that is assigned to a meeting location, 
such as a conference room, auditorium, or training room. 
Room mailboxes can be included as resources in meeting 
requests, providing a simple and efficient way of 
organizing meetings for your users.

In exchange 2003,
1.Mail-enabled user
2.Mailbox enabled user.
3.DL
4.Contact
5.Mail-Enabled public folder