For fledgling Exchange administrators to appreciate mailboxes in Microsoft Exchange, it's important to understand...
the idea of recipients -- and the varieties of Exchange recipient types. Microsoft considers a recipient to be any object in Active Directory that can send or receive messages from Exchange. Recipients are almost always people, but recipients can also be properly configured enterprise resources including equipment -- devices such as computers, projectors or conference lines -- or facilities such as conference rooms.
Generally, there is little real difference between recipients; every recipient ultimately sends or receives messages. But distinguishing Exchange recipient types is an excellent way to organize and manage recipients using tools such as the Exchange Administration Center and Exchange Management Shell. For example, a room recipient might display the room's occupancy capacity, but that detail doesn't apply to user recipients.
Microsoft Exchange Server 2016 is designed to accommodate 17 recipient types which primarily define individuals, groups and resources. However, administrators will typically encounter five Exchange recipient types more frequently than others.
The User Mailbox is perhaps the most common recipient type, and every Exchange user within the organization will receive a mailbox containing email messages along with various other items including calendar data, contacts, tasks and documents. Similarly, resource mailboxes such as Room Mailbox or Equipment Mailbox recipients represent enterprise resources, such as meeting rooms, projectors, conference lines and other business assets. Creating and configuring resource mailboxes allows users to ping resources for things such as meeting scheduling. For example, a manager might email meeting invitations to a team and include messages to resource mailboxes to schedule those items for the appointed meeting date and time.
Not all Exchange recipients exist within an enterprise. A mail user recipient has a valid Active Directory Domain Services account but uses an external email address. Mail users can access resources within the enterprise like any other user mailbox recipient, but messages to mail user recipients are simply routed outside of the business to the external email address. Similarly, mail contact recipients detail people or businesses outside of the enterprise, and messages to contacts are routed to those external email addresses. The difference between mail users and mail contacts is that mail user recipients can use enterprise resources -- such as data storage, conference rooms and telephone lines -- where contacts do not have that access.
Beyond the five individual Exchange recipient types above, administrators will frequently encounter common group recipients including shared mailbox, mail-enabled security group, dynamic distribution group or even Exchange shared mailbox recipient types.
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