17 Recipients Types in Exchange 2010 !!! take a look

Microsoft Exchange Server 2010 includes several explicit recipient types. Each recipient type is represented by a unique icon in the Exchange Management Console (EMC) and a unique name in the RecipientTypeDetails property in the Exchange Management Shell. The use of explicit recipient types has the following benefits:

• At a glance, you can differentiate between various recipient types.
• You can search, sort, and filter by each recipient type.
• You can more easily perform bulk management operations for each recipient type.
• You can more easily view recipient properties because the EMC uses the recipient types to render different property pages. For example, the resource capacity is displayed for a conference room mailbox, but isn't present for a user mailbox.

Dynamic distribution group
A distribution group that uses recipient filters and conditions to derive its membership at the time messages are sent.

Equipment mailbox A resource mailbox that's assigned to a non-location specific resource, such as a portable computer projector, microphone, or a company car. Equipment mailboxes can be included as resources in meeting requests, providing a simple and efficient way of utilizing resources for your users.

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

Linked mailbox A mailbox that's assigned to an individual user in a separate, trusted forest.

Mail contact A mail-enabled Active Directory contact that contains information about people or organizations that exist outside the Exchange organization. Each mail contact has an external e-mail address. All messages sent to the mail contact are routed to this external e-mail address.

Mail forest contact A mail contact that represents a recipient object from another forest. Mail forest contacts are typically created by Microsoft Identity Integration Server (MIIS) synchronization.
Important:
Mail forest contacts are read-only recipient objects that are updated only through MIIS or similar custom synchronization. You can't use the EMC or the Shell to remove or modify a mail forest contact.
Mail user A mail-enabled Active Directory user that represents a user outside the Exchange organization. Each mail user has an external e-mail address. All messages sent to the mail user are routed to this external e-mail address.
A mail user is similar to a mail contact, except that a mail user has Active Directory logon credentials and can access resources.

Mail-enabled non-universal group
A mail-enabled Active Directory global or local group object. Mail-enabled non-universal groups were discontinued in Exchange Server 2007 and can exist only if they were migrated from Exchange 2003 or earlier versions of Exchange. You can't use Exchange 2010 to create non-universal distribution groups.


Mail-enabled public folder An Exchange public folder that's configured to receive messages.

Mail-enabled universal distribution group
A mail-enabled Active Directory distribution group object that can be used only to distribute messages to a group of recipients.

Mail-enabled universal security group
A mail-enabled Active Directory security group object that can be used to grant access permissions to resources in Active Directory and can also be used to distribute messages.

Microsoft Exchange recipient A special recipient object that provides a unified and well-known message sender that differentiates system-generated messages from other messages. It replaces the System Administrator sender used for system-generated messages in earlier versions of Exchange. To learn more, see Understanding the Microsoft Exchange Recipient.

Room mailbox A resource mailbox that's 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.

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

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

Remote mailbox New in Exchange 2010, a remote mailbox consists of a mail-enabled user that exists in the on-premises Active Directory and an associated mailbox that exists in the cloud-based service.

Linked user New in Exchange 2010, a linked user is a user that resides in one forest while their mailbox resides in another forest.

Further info http://technet.microsoft.com/en-us/library/bb201680.aspx

Enjoy!

-Dario