In Exchange Server terminology and “recipient” is any object in the Active Directory environment that Exchange is able to send email messages to.
Each type of recipient in an Exchange Server environment has a different purpose and set of capabilities relating to its specific type. Although there are a large number of different recipient types they fall under a few broad categories.
Mailbox recipients come in several different types for different tasks or purposes.
User – this is the most common type of mailbox that is associated with a user in the same Active Directory forest as the Exchange organization.
Linked Mailbox – this is similar to a regular User mailbox however a Linked Mailbox is associated with a user in a different Active Directory forest to the local forest where the Exchange organization resides.
Linked User – this is similar to the Linked Mailbox only the association between user and mailbox is reversed, with a user in the local forest associated with a mailbox in a remote Exchange organization.
Shared – although each mailbox has a 1:1 association with a user object, a shared mailbox is one that is configured to allow multiple users to access it (for example a Help Desk mailbox).
Resource – resource mailboxes come in two types, Room and Equipment. Each is most often used with the calendaring features of Exchange to allow booking of meeting rooms or pool equipment. The main difference between the two is that rooms are typically fixed location whereas equipment is portable.
Legacy – this refers to any mailbox that still resides on an Exchange 2003 server, and only applies during the transitional period from Exchange 2003 to 2010. Once a mailbox is moved from 2003 to 2010 it becomes either a User or Shared mailbox, depending on who has permissions to access it.
Exchange Server 2010 no longer supports Global or Domain Local groups, unless they were already mail-enabled in Exchange 2003 prior to deploying Exchange 2010. All mail-enabled groups in Exchange 2010 must be Universal groups.
Dynamic – a dynamic group is one in which the membership is not statically assigned, rather it is based on a query that is assessed at the time messages are sent to the group. For example, a dynamic group may be one for all users that exist on a specific database, which can change frequently in some organizations.
Distribution – a distribution group is a Universal Group that is mail-enabled, but cannot be used for purposes other than email.
Security – a security group is a Universal Group that is mail-enabled, and can be used for both sending email to a group of recipients as well as assigning permissions to other objects and resources.
Aside from mailboxes and groups there are a few other types of recipients.
Public Folders – a public folder can be mail-enabled and receive email messages.
Contact – contacts are usually created as a way of providing centralized access to a list of email addresses of external parties that do not have accounts in the Active Directory forest.
Mail User – a mail user is similar to a contact in the receiving mailbox is external to the Exchange organization, however the user object itself exists within the Active Directory.
As you can see even though it may appear that there is a complex array of recipient types in Exchange Server 2010, each serves a specific purposes and when deployed correctly they are useful and efficient ways to manage your email environment.