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Distribution mailbox
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Room mailbox
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Legacy mailbox
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User mailbox
EXPLANATION
The following are all Exchange Recipient types in MS Exchange.
User mailbox: This mailbox is created for an individual user to store
mails, calendar items, contacts, tasks, documents, and other business
data.
Linked mailbox: This mailbox is created for an individual
user in a separate, trusted forest. For example AD account is created in
A.COM and Mailbox is created in B.COM Exchange Server.
Shared
mailbox: This mailbox is not primarily associated with a single user and
is generally configured to allow logon access for multiple users.
Legacy mailbox: This mailbox is resides on a server running Exchange Server 2003 or Exchange 2000 Server. See here:
https://support.microsoft.com/en-us/help/931747/a-mailbox-that-is-located-on-an-exchange-server-2007...Room
mailbox: This mailbox is created for a meeting location, such as a
meeting or conference room, auditorium, or training room. When we create
this mailbox, by default a disabled user object account is created.
Equipment
mailbox: A resource mailbox is created for a non-location specific
resource, such as a portable computer projector, microphone, or a
company car. When we create this mailbox, by default a disabled user
object account is created. Equipment mailboxes provide a simple and
efficient way for users to use resources in manageable way.
SOURCE
https://technet.microsoft.com/en-us/library/bb201680(v=exchg.150).aspx
