A recipient is any mail-enabled object in Active Directory to which Microsoft Exchange can deliver or route messages.
Exchange includes several recipient types. Each recipient type is identified in the Exchange admin center and has a unique value in the Recipient Type Details property in the Exchange Management.
Breakdown of Office 365 Exchange Recipient Types
Mailboxes
Mailboxes are the most common recipient type used by information workers in an Exchange organization. Each mailbox is associated with an Active Directory user account. The user can use the mailbox to send and receive messages, and to store messages, appointments, tasks, notes, and documents. Mailboxes are the primary messaging and collaboration tool for the users in your Exchange organization.
The Types of Mailboxes include:
User mailboxes
A mailbox that's assigned to an individual user in your Exchange Online organization. A mailbox contains the user's email messages, calendar items, contacts, tasks, and other important business data.
Shared mailboxes
A mailbox that's designed for access by multiple users.
Shared mailboxes do not require licenses in Exchange Online.
Resource mailboxes
A type of 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.
Equipment mailboxes
A type of 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.
Groups
Distribution group
Distribution groups, also called distribution lists, send emails or meeting requests to multiple people. Distribution groups come in handy for project teams or committees since you can send messages without having to add every email address each time.
Dynamic distribution group
A dynamic distribution group uses recipient filters and conditions to periodically calculate the membership of the group. It’s a query-based group unlike a dynamic group in Azure where we can integrate this with any of the attributes such as Office, City, and Custom Attribute and add members to this group.
Microsoft 365 group
Microsoft 365 groups (formerly known as Office 365 groups), are used for collaboration between teams, both inside and outside your company, by providing group email and a shared workspace for conversations, files, and calendars.
For email, the benefit of a Microsoft 365 group over traditional groups is: that the email history of the group is preserved. If a new user joins an old Microsoft 365 group, the entire email history of the group is available to them.
Mail-enabled security group
Like a distribution group, a mail-enabled security group provides a single point of contact for delivering emails to the members of the group. However, a mail-enabled security group is also a security principal, which means you can assign permissions to the group that affect all group members who are also security principals (user mailboxes, mail users, other mail-enabled security groups, etc.
Mail contact
A mail contact contains information about a person who's outside of your Exchange Online organization. A mail contact has an external email address, but the mail contact is visible in your organization's shared address book (also known as the global address list or GAL) and other address lists.
Mail Contacts are nothing, but directory service objects that contains the information about the user that exist outside/external of the exchange organization.
Each contact will have an external email address and the email sent to the contact will route to this external email address.
Mail-enabled
A mail user (also known as a mail-enabled user) is similar to a mail contact in that it represents a user with an external email address and is visible in your organization's shared address book and other address lists. However, a mail user also has a user account in your organization, and you can assign permissions to the mail user.
A mail-enabled user is an Active Directory account that represents a user outside of the Exchange organization. Each mail user has an external email address and the emails sent to the mail user are routed to this external email address.
NOTE: A mail user is similar to a mail contact; however, a mail user has Active Directory logon credentials and can access resources within the exchange organization, but a mail contact cannot.