Simple definition
An Archive Mailbox is an additional archive mailbox associated with an Exchange Online mailbox to separate older messages from the primary mailbox.
Technical definition
The online archive provides a separate store that is visible in supported clients. It can be populated manually or through retention policies and tags that move items when defined conditions are met.
How it works / role
After it is enabled, the user sees an archive separate from the primary mailbox. Retention processing evaluates messages and can move items that match an archive rule. Available capabilities depend on tenant configuration and applicable rights or licensing.
What is it used for?
Keep older messages in a managed archive while reducing the amount of content retained in the primary mailbox.
Practical example
A policy automatically moves messages that meet the organization’s age criteria into the archive; the user can still access them from Outlook or Outlook on the web.
Common issues
- The mailbox does not have the required archive capability or license
- The archive is not enabled or is not visible in the client
- A retention policy or tag is missing, not applied, or misunderstood
- Archiving is confused with legal retention or backup
Related terms
Exchange Online · Microsoft 365 Retention Policy
Key takeaway: An Archive Mailbox is managed messaging storage; its behavior depends on activation and the retention policies that actually apply.