The people and resources that send and receive messages are the core of any messaging and collaboration system. In an Exchange Online organization, these people and resources are referred to as recipients. A recipient is any mail-enabled object to which Exchange Online can deliver or route messages. Exchange includes several explicit recipient types, namely:
| Recipient Type |
Description |
| 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 resource that's not location-specific, 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 using resources for your users. |
| 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 email address. All messages sent to the mail contact are routed to this external email 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. |
| Mail user |
A mail-enabled Active Directory user that represents a user outside the Exchange organization. Each mail user has an external email address. All messages sent to the mail user are routed to this external email address. |
| Mail-enabled public folder |
An Exchange public folder that's configured to receive messages. |
| Distribution groups |
A distribution group is a mail-enabled Active Directory distribution group object that can be used only to distribute messages to a group of recipients. |
| Mail-enabled security group |
A mail-enabled security group is an Active Directory universal security group object that can be used to assign 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. |
| 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 comprised of an Exchange mailbox to store email messages and a SharePoint site to store documents. Users can access both email messages and documents using the same client interface. For more information, see Site mailboxes. |
| 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. |
| Office 365 mailbox |
In hybrid deployments, an Office 365 mailbox consists of a mail user that exists in Active Directory on-premises and an associated cloud mailbox that exists in Exchange Online. |
| Linked user |
A linked user is a user whose mailbox resides in a different forest than the forest in which the user resides. |
| Discovery mailbox |
Discovery mailboxes are used as target mailboxes for In-Place eDiscovery searches in the Exchange admin center (EAC). |
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To know which recipient types are supported by the Exchange Online organization and the number of recipients configured for each type, administrators can use the EXOReciTypeTest test. For recipient types that are mailboxes, the test additionally reports the total size of mailboxes of that type and the total number of emails in those mailboxes. This way, administrators can easily manage recipients and track the growth in size of the recipient mailboxes.