Types of Exchange Server Recipients.

Exchange Server recipients are any objects within the Active Directory Domain Services (AD DS) forest that have been configured with an email address. When AD DS objects are configured with an email address, they appear in the Global Address List (GAL). Exchange Server 2016 supports the following recipient types:

User mailboxes:- A mailbox that you assign to an individual user in your Exchange organization. This is the most common type of recipient in Exchange Server 2016.

Mail contacts:- Contacts that contain information about people or organizations that exist outside an Exchange Server organization and that have an external email address. Exchange Server routes all messages sent to the mail contact to this external email address. 

Mail users:- Users who have an AD DS user account but have an external email address.
All messages sent to the mail user are routed to this external email address. A mail user is similar to a mail contact, except that a mail user has an AD DS user account with a security identifier (SID). This allows the user account to access resources in the AD DS environment. 

Resource mailboxes (room mailboxes and equipment mailboxes). A resource mailbox is configured for objects such as meeting rooms, or resources such as a projector. You can include resource mailboxes as resources in meeting requests, which provides a simple and efficient way to schedule resource usage. 

Shared mailboxes. A mailbox that is used by multiple users rather than one primary user. Organizations often use shared mailboxes to provide services such as sales, help desk, or general information requests. 

Mail-enabled security and distribution groups. You can use a mail-enabled AD DS security group object to grant access permissions to AD DS resources, and you also can use it to distribute messages. You can use a mail-enabled AD DS distribution group object to distribute messages to a group of recipients. 

Dynamic distribution groups. A distribution group that uses a Lightweight Directory Access Protocol (LDAP) query with recipient filters and conditions to derive its membership at the time messages are sent. 

Linked mailboxes. User mailboxes that are associated with specific users in a separate, trusted forest. When you create a linked mailbox, a disabled user account is created in the Exchange organization, and a user account from a trusted forest is given access to the mailbox. 

Remote mailboxes. Mailboxes that are located in the Exchange Online environment. In a hybrid Exchange Server 2016 deployment, you can create and manage remote mailboxes in the Exchange Online environment by using the Exchange Admin Center and the Exchange Management Shell. 

Site mailboxes. Mailboxes that include both an Exchange Server mailbox and a Microsoft SharePoint site. With site mailboxes, messages are stored in the mailbox, whereas documents are stored on the SharePoint site.