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Fix Exchange Online Public Folder Mailbox Over Storage Limit
Question details
Users encounter storage errors in Exchange Online public folders because the hosting mailbox has exceeded its quota, and AutoSplit is not triggering as expected.
- Product
- Exchange Online (Microsoft 365)
- Device & OS
- not provided
- Scenario
- Managing public folder storage quotas and attempting to trigger AutoSplit via PowerShell.
- Observed behavior
- Public folders report storage limit errors despite individual folders being under the limit, caused by the hosting public folder mailbox exceeding its quota with AutoSplit failing or delaying.
Ensure you have Microsoft 365 global administrator or Exchange administrator privileges before attempting to review mailbox quotas or submit support requests.
Review Mailbox Quotas and Troubleshoot AutoSplit Delays
Identify common bottlenecks preventing the AutoSplit feature from balancing your public folder storage.
Exchange Online manages public folder mailboxes through a feature called AutoSplit. AutoSplit performs automatic public folder moves based on quota usage and creates mailboxes based on hierarchy connections. When this process stalls, the hosting mailbox can exceed its limit.
Review your environment for conditions that delay AutoSplit, such as sudden data growth, exceptionally large individual folders, or retained moved items.
Use Exchange Online PowerShell to check for any Mailbox Replication Service (MRS) problems or hierarchy sync issues.
Consult the official Microsoft AutoSplit documentation to verify your public folder distribution and move status align with expected parameters.
Open a Microsoft 365 Service Request
If AutoSplit still fails to operate and the mailbox remains over its limit, escalate the issue to Microsoft Support.
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Frequently Asked Questions
What is AutoSplit in Exchange Online?
AutoSplit is an automated feature in Exchange Online that manages public folder mailboxes by moving folders to new mailboxes when storage quotas are reached, preventing any single mailbox from exceeding its capacity.
Why isn't AutoSplit triggering when I move folders using PowerShell?
AutoSplit can be delayed by sudden data growth, exceptionally large individual folders, Mailbox Replication Service (MRS) problems, or issues with retained moved items that tie up storage space.
What should I do if my public folders are under the limit but the hosting mailbox is full?
This indicates that AutoSplit is failing to distribute the load to new mailboxes. You should verify your Mailbox Replication Service (MRS) status and, if it remains unresolved, submit a service request through the Microsoft 365 admin center.
