Estimated reading: 5 minutes 63 views Contributors Greg Buckles


The Microsoft 365 mailbox is the primary storage container for communication content generated in Outlook, Teams, Viva Engage (Yammer), Planner, Streams and other partner applications. It is the central repository where original or compliance copy emails, chat, calendar entries, tasks, and other communication related data are retained and managed. Microsoft consolidated all the visible and hidden communication information into a mailbox database managed by Exchange Online to maximize user access and search performance. As discussed in the Compliance Architecture page, every licensed user or M365 Group (think SharePoint/Teams sites and channels) is provisioned with a mailbox and SharePoint store.

Exchange Mailbox Containers:

  1. User Mailbox: A mailbox assigned to an individual user in your Exchange organization. It contains the user’s email messages, calendar items, contacts, tasks, and other important business data.
  2. Group mailboxes: Created to contain communications and other M365 Group
  3. Shared Mailbox: Designed for access by multiple users, shared mailboxes do not require licenses in Exchange Online. They serve as a collaborative mailbox for teams or departments.
  4. Mail Contact: Represents a person or organization outside your Exchange organization. A mail contact has an external email address and is visible in your organization’s shared address book (also known as the global address list or GAL).
  5. Mail User: Similar to a mail contact, a mail user represents a user with an external email address. However, a mail user also has a user account in your organization, and you can assign permissions to the mail user. Mail users do not require licenses in Exchange Online.
  6. Room Mailbox: A type of resource mailbox assigned to a meeting location, such as a conference room, auditorium, or training room. Room mailboxes can be included as resources in meeting requests.
  7. Equipment Mailbox: Another type of resource mailbox, it’s assigned to a resource that’s not location-specific, such as a portable computer, projector, microphone, or a company car. Equipment mailboxes can also be included as resources in meeting requests.
  8. Distribution Group (Distribution List): Provides a single point of contact for delivering email to the members of the group. You can create and manage distribution groups.
  9. Mail-Enabled Security Group: Similar to a distribution group, a mail-enabled security group also provides a single point of contact for delivering email to its members. However, it’s associated with security permissions.
  10. Guest Mailboxes: Guest account is an external user who isn’t part of your organization. These guests can access and collaborate on resources within your M365 environment. They have their own inbox within M365 Groups, and messages sent to the group by other members appear in their inbox. Guest mailboxes are NOT included in global searches by default in Purview eDiscovery (needs to be retested).
  11. Shared Teams channels: A mailbox and SharePoint site storage are created automatically for Teams channels because they are M365 Groups.
  12. Inactive mailboxes: When a M365 user or group account is removed the mailbox data is retained for 30 days by default unless any content is under legal hold or is labeled with specific retention policies. A list of inactive mailboxes can be viewed in the Purview Data Governance module.

Mailbox Item types:

  • contacts
  • docs
  • email
  • externaldata
  • fax
  • im
  • journals
  • meetings
  • microsoftteams
  • notes
  • posts
  • rssfeeds
  • tasks
  • voicemail

Searchable properties associated with the different item types will be covered in the Purview search page under development.

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