Why do we need to archive mail box?

Enabling an Online Archive creates a separate archive mailbox; messages moved there remain stored and accessible, and they are not permanently deleted unless a separate retention policy explicitly applies a deletion action.

What happens to archived email

• Email can be moved manually or automatically from the primary mailbox to the Online Archive.
• You can open the Online Archive in Outlook or Outlook on the web, browse its folders, and move or copy messages between it and the primary mailbox. It remains part of your Exchange Online mailbox for retention, hold, and compliance purposes.^1^
• Under Exchange Online’s default retention policy, items that are two years old are moved from the primary mailbox to the archive mailbox. Your administrator can customize this timing.
• Move to Archive and Permanently Delete are different retention actions. Moving a message to the archive does not delete it.^2^

Deletion can still happen later if your organization has configured a deletion policy. Such a policy can apply to both the primary and archive mailboxes. Ask your administrator what retention and deletion periods apply before relying on the archive as permanent storage.

What to do about the “archive almost full” warning

Because your Online Archive already reports that it is almost full, send the warning to your Microsoft 365 or Exchange administrator and ask them to:

1. Check the visible and Recoverable Items usage of the archive mailbox.
2. Review your assigned license and archive quota.
3. Confirm whether auto-expanding archiving is enabled.
4. Review retention and deletion policies applied to your mailbox.

Only an administrator can enable auto-expanding archiving. For supported licenses, it automatically adds archive storage as the archive approaches its quota, although provisioning additional capacity can take up to 30 days.^3^

Auto-expanding capacity is license-dependent:

• Supported licenses generally provide expansion up to 1.5 TB.
• The increase to 3 TB applies only to the eligible E5 and other listed Purview-capable licenses after its October 2026 rollout; other supported licenses remain limited to 1.5 TB.
• For eligible E5 users, Records Management must remain enabled in the user’s license settings.^3^

The administrator can use Microsoft Learn’s Enable auto-expanding archiving procedure linked from that documentation. If the archive has reached its maximum capacity, the organization must delete content that no longer has business or legal value under an approved retention policy or address the applicable licensing.

Finding archived messages

In Outlook, expand Online Archive – your mailbox name in the folder pane. In Outlook on the web, expand the Online Archive entry in the folder list. Archived content remains available there, including content placed in automatically created subfolders when additional archive storage is provisioned.^3^

Do not confuse Online Archive with Outlook’s ordinary Archive folder. Clicking Outlook’s Archive button only moves mail to a folder inside the primary mailbox and does not reduce the primary mailbox’s storage usage.^4^

References

1. Learn about archive mailboxes
2. Retention tags and retention policies in Exchange Online
3. Learn about auto-expanding archiving
4. Archive in Outlook for Windows | Microsoft Support