What Is Exchange Online Archiving?
Exchange Online Archiving (EOA) is Microsoft's in-place archive mailbox feature for Exchange Online users. Each licensed user gets a second mailbox - separate from their primary inbox but accessible from Outlook - that holds older or lower-priority email and frees up space in the primary mailbox. Microsoft handles the underlying storage, expansion, and indexing; administrators define which messages move there using retention policies.
EOA is widely confused with two adjacent things: it is not a backup, and it is not a long-term archive for departed-employee data. Both of those are gaps EOA does not cover, and both are common reasons enterprises end up looking for something beyond EOA - including Chipmunk for departed-user mailbox preservation.
This guide explains what Exchange Online Archiving actually does, what it costs, how it is licensed, what it does not cover, and where third-party tools fit alongside it.
What Microsoft Exchange Online Archiving Costs
Exchange Online Archiving is included in several Microsoft 365 enterprise plans and available as a paid add-on for others. At a high level:
- Included with: Microsoft 365 E3, E5, A3, A5, G3, G5, and Office 365 equivalents.
- Add-on for: Microsoft 365 Business plans, E1, F1, F3, standalone Exchange Online Plan 1, and most other lower-tier plans.
- Standalone: Exchange Online Archiving licences can be purchased independently and attached to mailboxes that don't already include the feature.
Microsoft publishes current Exchange Online Archiving pricing on its Microsoft 365 plan comparison pages, and pricing changes periodically. As of recent pricing, the add-on has been a low single-digit USD figure per user per month, but always check the live Microsoft pricing page before sizing a deployment.
For most enterprises on E3 or higher, EOA is already bundled into the licence they pay for. The cost question only matters for organisations on F-line, E1, or Business plans deciding whether to add it.
How Exchange Online Archiving Works
Once enabled on a mailbox, EOA creates an in-place archive that appears in Outlook as a separate folder tree below the primary mailbox. From the user's perspective:
- The archive looks and behaves like a regular mailbox folder.
- Search across "All Mailboxes" includes archive content.
- Older or rule-matched messages move into the archive automatically based on the Default MRM Policy or a custom retention policy.
- Auto-expanding archive grows the archive size automatically as content accumulates - starting around 50 GB and expanding up to roughly 1.5 TB, subject to Microsoft's fair-use limits.
Administrators control what moves to the archive via retention tags and retention policies in the Microsoft 365 compliance centre. eDiscovery, Litigation Hold, and In-Place Hold all apply to archive content the same way they apply to primary mailboxes.
What Exchange Online Archiving Doesn't Cover
EOA is purpose-built for one thing: giving active users a larger usable mailbox without bloating the primary. It is not a general-purpose archiving solution, and there are four well-known gaps that catch enterprises out:
1. Departed-user data
The archive mailbox lives inside the user's licence. When that user leaves and you reclaim the licence, the archive mailbox is removed on Microsoft's standard timeline - unless a retention policy or hold preserves it explicitly. Many tenants discover this only when legal asks for an ex-employee's mailbox months later and finds nothing left to discover.
This is the gap Chipmunk is built for: it automatically detects disabled accounts and archives the user's mailbox (along with OneDrive and Teams content) to your own Azure Blob Storage before Microsoft's deletion timers expire. The licence can then be reclaimed cleanly without losing the data. See the full Microsoft 365 departed-user archiving guide for the offboarding workflow.
2. Backup and recovery
EOA is not a backup. It does not protect against ransomware, malicious admin actions, mass accidental deletion outside retention windows, or corruption events. Microsoft's Shared Responsibility Model is clear that customer data protection sits with the customer; high availability and disaster recovery are not the same as backup. See Microsoft 365 Backup: What MS 365 Covers and What It Doesn't for the full backup-vs-archive picture.
3. SharePoint and OneDrive storage
EOA only applies to Exchange mailboxes. It does nothing for the other half of the Microsoft 365 storage problem - SharePoint and OneDrive content - which for most enterprises is the larger and faster-growing cost. The SharePoint side is handled by Squirrel, which archives inactive SharePoint Online documents into Azure Blob Storage in your own subscription, leaving stub files for one-click restore.
4. Data sovereignty and exit strategy
EOA content lives inside Microsoft's cloud and is governed by Microsoft's licensing. If you need archive data in your own Azure subscription, under your own retention policies, with an independent exit path, EOA does not provide that. Third-party archivers like Chipmunk and Squirrel keep archived content in your tenant, with your encryption keys, so the data outlives the Microsoft licence.
Exchange Online Archiving vs Third-Party Archiving
These solve different problems and often work together rather than competing:
| Need | Exchange Online Archiving | Third-Party Archiving |
|---|---|---|
| Larger mailbox for active users | Yes - in-place archive | Not the primary purpose |
| Departed-user mailbox preservation | Limited (tied to licence) | Yes - Chipmunk archives before licence reclaim |
| SharePoint storage reduction | No | Yes - Squirrel for SharePoint Online |
| Data in customer-owned Azure storage | No | Yes - Chipmunk and Squirrel |
| Backup against ransomware/corruption | No | Yes (with a dedicated backup product) |
| Cost beyond bundled E3/E5 licences | Add-on per user per month | One-time architecture, infrastructure runs in your tenant |
For most large enterprises, the right pattern is: EOA on for active users (it's already in your E3 anyway), Chipmunk for departed-user data preservation, Squirrel for SharePoint storage growth, and a separate backup strategy for ransomware and corruption recovery.
When Exchange Online Archiving Is Enough
EOA on its own can be sufficient when:
- The tenant is small and turnover is low.
- The only concern is mailbox capacity, not long-term preservation after offboarding.
- Compliance requirements are met by retention policies inside Microsoft 365.
- There is no business case for keeping archive data outside Microsoft's cloud.
When You Need Something Beyond Exchange Online Archiving
Enterprises typically look past EOA when:
- Employee turnover is significant and offboarding data preservation is a recurring legal or HR requirement.
- Compliance teams require independent archives that are not tied to active Microsoft 365 licences.
- SharePoint storage is also growing and the Preservation Hold Library is silently driving costs up.
- Data sovereignty matters - the archive must live in customer-controlled Azure storage, not inside Microsoft's tenant.
In those cases, Chipmunk handles the departed-user side, Squirrel handles SharePoint archiving, and EOA continues to handle active-user mailbox capacity. The three are complementary, not competing.
Frequently Asked Questions About Exchange Online Archiving
What is Exchange Online Archiving? Microsoft's in-place archive mailbox feature for Exchange Online users. Each licensed user gets a second mailbox accessible from Outlook for old or low-priority email, governed by retention policies and auto-expanding storage.
Is Exchange Online Archiving included in my Microsoft 365 licence? Included with Microsoft 365 E3, E5, A3, A5, G3, G5, and Office 365 equivalents. Available as a paid add-on for E1, F1, F3, Business plans, and standalone Exchange Online plans.
How much does Exchange Online Archiving cost? Pricing changes - check Microsoft's current Exchange Online Archiving plan page. The standalone add-on has historically been a low single-digit USD per user per month. For most enterprises on E3 or higher, EOA is already bundled.
What is the storage limit on Exchange Online Archiving? Auto-expanding archive starts at around 50 GB and grows automatically up to roughly 1.5 TB per mailbox, subject to Microsoft's fair-use policies. Microsoft adjusts these limits over time.
Does Exchange Online Archiving back up my data? No. EOA is an archive mailbox, not a backup. It does not protect against accidental deletion outside retention windows, ransomware, or licence loss. Backup is a separate concern - see the Microsoft 365 backup guide.
What happens to a user's archive mailbox when they leave? The archive mailbox is tied to the user's licence. Once the licence is removed, the archive is deleted on Microsoft's standard timeline unless a retention policy or hold preserves it. This is the most common EOA gap - and the problem Chipmunk solves by preserving the mailbox to your own Azure Blob Storage before the licence is reclaimed.
Can I use Exchange Online Archiving without an E3 licence? Yes - as a paid add-on. EOA add-on licences attach to mailboxes that don't already include the feature.
What is the difference between Exchange Online Archiving and Microsoft Purview retention? EOA gives users a separate archive mailbox for everyday use. Purview retention enforces compliance rules - what must be kept or deleted, and for how long. They commonly work together: Purview sets the policy, EOA gives the user-facing archive mailbox.
Does Exchange Online Archiving help with SharePoint storage? No. EOA only applies to Exchange mailboxes. For SharePoint storage growth - particularly the Preservation Hold Library problem - Squirrel is the equivalent purpose-built tool, archiving SharePoint content to Azure Blob Storage in your own subscription.
Is Microsoft Exchange Online Archiving the same as a journaling solution? No. EOA gives users an additional mailbox for old content. Journaling captures a copy of every message sent or received for compliance - a different feature with different licensing requirements. Microsoft positions Purview Communication Compliance for that use case.
Related guides on Microsoft 365 archiving
- Microsoft 365 departed-user archiving - the full guide to preserving an ex-employee's data before Microsoft's deletion timers expire.
- Chipmunk overview - the product built for departed-user mailbox, OneDrive and Teams preservation.
- Microsoft 365 backup: what MS 365 covers and what it doesn't - the broader backup-vs-archive picture.
- What does archiving mean? - the foundational guide to data archiving in the enterprise.
- Squirrel - SharePoint Online archiving - the SharePoint side of the Microsoft 365 archiving problem.
Talk to SmiKar About Microsoft 365 Archiving Beyond EOA
If Exchange Online Archiving is already running in your tenant and you have gaps - departed-user preservation, SharePoint storage growth, or data sovereignty requirements - the Squirrel and Chipmunk team can help you scope the right approach.
