How to Properly Set Up Auto-Expanding Archives in Office 365 (Step-by-Step Guide)
Managing large mailboxes in Office 365 (now Microsoft 365) can be a major challenge for IT admins. Fortunately, auto-expanding archive mailboxes make it easy to provide users with virtually unlimited email storage — without manual intervention.
In this reddit guide, we’ll cover exactly how to archives that optimize your email archiving to ensure that you never have to manually create a PST again.
What Is an Auto-Expanding Archive in Office 365?
An auto-expanding archive is a special type of mailbox that automatically archives your email to ensure you don’t lose email deliverability due to hitting the standard 100 GB mailbox limit
Key facts:
- The archive starts with 100 GB of space.
- As users fill it, additional archive storage is provisioned automatically (in 50 GB increments).
- Microsoft states that archives can grow up to 1.5 TB or more per mailbox.
- Auto-expanding archives only work if you have retention policies in place that move older emails from the primary mailbox into the archive.
How to Set Up Auto-Expanding Archives in Office 365
1. Enable the Archive Mailbox
From Exchange Admin Center (EAC):
- Go to https://admin.exchange.microsoft.com.
- Click Recipients > Mailboxes.
- Select a mailbox.
- Under Mailbox features, click Enable archive.
Or via PowerShell:
Connect-ExchangeOnline -UserPrincipalName admin@yourdomain.com
Enable-Mailbox -Identity user@yourdomain.com -Archive
2. Enable The Auto-Expanding Archive
Once the archive is active, you must enable the auto-expansion to occur on a per mailbox basis:
Enable-Mailbox -Identity user@yourdomain.com -AutoExpandingArchive
3. Create and Apply a Retention Policy that Automatically Moves Email to the Archive
Auto-expanding alone doesn’t move emails. You need a retention policy that defines when to move old emails into the archive. While the email will automatically archive, you have to define after how many days an email will automatically archive.
Create a Retention Tag (Move to Archive After 3 Years):
New-RetentionPolicyTag -Name "3 Year Archive Tag" `
-Type All `
-RetentionEnabled $true `
-AgeLimitForRetention 1095 `
-RetentionAction MoveToArchive
Create a Retention Policy Using That Tag So That You Can See the Policy in the Admin Panel:
New-RetentionPolicy -Name "3 Year Archive Policy" `
-RetentionPolicyTagLinks "3 Year Archive Tag"
4. Assign the Retention Policy to Users
You must assign the retention policy to each user who needs auto-archiving.
Via PowerShell:
Set-Mailbox -Identity user@yourdomain.com -RetentionPolicy "3 Year Archive Policy"
Or via the Admin Center:
- Go to https://admin.exchange.microsoft.com.
- Navigate to Recipients > Mailboxes.
- Click the user’s mailbox.
- Under Mailbox policies, click Manage mailbox policies.
- Under Retention policy, choose your new Archive Policy from the dropdown.
- Save changes.
✅ Now, emails older than three years will automatically move into the archive mailbox.
5. Force Immediate Processing (Optional)
Normally, retention policies are enforced by the Managed Folder Assistant within 7 days. To speed it up:
Start-ManagedFolderAssistant -Identity user@yourdomain.com
This forces immediate processing of the mailbox.
How Users Find and Search Their Archived Emails
Once the archive mailbox is enabled, users often ask: “Where are my archived emails?” Here’s how it works:
A) How to Find Archived Emails
- Outlook Desktop App:
In the folder list (left-hand side), users will see a separate section labeled “Online Archive – [Mailbox Name]” below their main mailbox folders. - Outlook Web App (OWA):
In the left navigation pane, under folders, users will also see an Online Archive section listed below their primary mailbox. - Mobile Devices (Outlook Mobile App):
- Most mobile apps do not show the archive mailbox.
- Users must use Outlook Desktop or Outlook Web App to access archived messages.
B) How to Search Archived Emails Easily
Searching includes both the primary mailbox and the archive if users search correctly:
- Outlook Desktop:
- Click inside the Search bar.
- Choose “All Mailboxes” or “All Outlook Items” from the search scope dropdown.
- Outlook will search both the primary mailbox and the archive automatically.
- Outlook Web App (OWA):
- Use the main Search bar at the top.
- By default, it searches all folders across both the primary mailbox and the archive.
✅ Important Tip:
When searching, select “All Mailboxes” for broader searches — otherwise, you might not find results stored in the archive.
How Does Auto-Expanding Archive Affect Users?
- Outlook Desktop: Users see an “Online Archive” listed alongside their folders.
- Outlook Web App (OWA): Online Archive appears automatically.
- Mobile Devices: Archive mailbox typically not available on mobile apps.
- Searching: Works seamlessly when searching “All Mailboxes” or “All Items.”
Common Pitfalls to Avoid
- Just enabling the archive doesn’t move mail!
Always create and assign a retention policy. - Retention doesn’t act immediately.
It can take up to 7 days unless manually forced. - Mobile apps may not show the archive.
Inform users and suggest they use Outlook Desktop or Web for full access.
Final Thoughts
Setting up auto-expanding archives properly in Microsoft 365 ensures users never run out of mailbox space — while keeping primary mailboxes lightweight and easy to manage. By combining archive enablement, auto-expansion, and well-crafted retention policies, you can drastically reduce administrative overhead and improve user satisfaction.
Need help managing Office 365 archiving, compliance, or retention strategies? Contact our Office 365 experts today!
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