Microsoft documents archive mailbox limits for Exchange Online and Microsoft 365. Enabling an archive mailbox gives you 100 GB of extra space. Auto-expanding archiving adds up to 1.5 TB of storage and up to 3 TB on eligible licenses. Auto-archiving moves the oldest items once the primary mailbox passes 96 percent of its quota. Microsoft gives no steps for a full local Outlook archive file.
Full archive folder reports point to one of two places. Your local Outlook data file for archived mail has grown large. Your Exchange Online archive mailbox has reached its limit. Both places need different fixes.
This post addresses the second place. Facts follow the Microsoft Purview article Learn about archive mailboxes.
What is an Archive Mailbox?
Archive mailbox is an additional mailbox linked to your primary mailbox. Microsoft also calls it In-Place Archiving. Outlook and Outlook on the web store messages in it. You can move or copy messages between the two mailboxes.
Content search, Microsoft 365 retention and Litigation Hold treat both mailboxes as one mailbox.
How Much Storage does an Archive Mailbox have?
| Item | Limit stated by Microsoft |
|---|---|
| Archive mailbox after you enable it | Up to 100 GB of extra storage |
| Auto-expanding archiving | Up to 1.5 TB of additional archive storage |
| Auto-expanding archiving on eligible licenses | Up to 3 TB, with rollout from mid-October 2026 |
Eligible Exchange Online mailboxes only get the 3 TB limit. Microsoft names Microsoft 365 E5, A5 and G5 among the licenses. Every other license remains at 1.5 TB.
Microsoft starts auto-expanding archiving after the first 100 GB is in use. Microsoft explains its setup in a separate article.
How does Auto-Archiving Move Items?
- Threshold. Microsoft states that the oldest items move to the archive when the primary mailbox passes 96 percent of its quota. This is the default.
- Requirement. Auto-archiving needs an archive mailbox with free capacity.
- Exclusions. Items tagged Never Move to Archive and recently received items remain in the primary mailbox.
- Tenant setting. Organizations can edit the threshold. Setting it to 100 switches auto-archiving off for the tenant.
Microsoft also assigns a default retention policy to every mailbox. It moves items two years old or older to the archive. It moves Recoverable Items older than 14 days into the archive Recoverable Items folder.
What if the Archive Mailbox is Full?
Microsoft lists the limits above and no repair steps. Microsoft does not say what Outlook shows when the archive reaches 100 GB. It does not say how to free space.
Two facts from the article point to the next step. Auto-expanding archiving adds storage after 100 GB. Microsoft explains how to enable it in its article on that feature.
What does Microsoft Not Say?
- Default size of the primary mailbox quota.
- Permission rules for enabling an archive mailbox. Microsoft points to an administrator guide.
- Any fix for an Outlook archive file (.pst) that has grown too large on your computer.
Related reading: merge Outlook archive folders and fix a full Outlook mailbox.
Frequently Asked Questions
How Big is an Outlook Archive Mailbox?
Microsoft states up to 100 GB of extra storage after you enable the archive. Auto-expanding archiving then adds up to 1.5 TB.
What is the 3 TB Archive Limit?
Microsoft raises the auto-expanding maximum from 1.5 TB to 3 TB for eligible Exchange Online mailboxes. Rollout starts in mid-October 2026.
When does Auto-Archiving Start?
Microsoft states that it starts when the primary mailbox passes 96 percent of its quota. This is the default threshold.
Does Auto-Archiving Move Every Item?
No. Items tagged Never Move to Archive and recently received items remain in the primary mailbox.
Can I Enable the Archive Myself?
Microsoft does not say. Its article points to a separate guide written for Exchange and compliance administrators.
Key Takeaways
- Archive mailboxes add up to 100 GB of space.
- Auto-expanding archiving adds up to 1.5 TB, with up to 3 TB on eligible licenses.
- Auto-archiving starts at 96 percent of the primary mailbox quota by default.
- Microsoft gives no repair steps for a full archive in this article.
- Local archive files that grow too large need a different guide.
Sources and References
Facts follow Microsoft Learn.
- Learn about archive mailboxes (Microsoft Learn) learn.microsoft.com