Auto-Archiving in Exchange Online to Prevent Mail Flow Disruptions

Managing mailbox storage in Exchange Online has always been a challenge for Microsoft 365 admins. Mailboxes can quickly reach their storage limits, disrupting email flow and user productivity.

Traditional retention policies and time-based archiving only acts after a specified period, leaving mailboxes vulnerable to sudden data surges. Auto-Archiving in Exchange Online solves this by automatically moving the oldest mailbox items to the archive when usage exceeds 90%, keeping email flow smoothly without interrupting business collaboration.

Rollout Timeline

• Public Clouds: October 15, 2025
• Government Clouds: November 2025

Auto-Archiving is being rolled out automatically for tenants with archive-enabled mailboxes.

It helps keep storage under control and ensures uninterrupted email access across the organization.

Auto-Archiving adds a threshold-based layer on top of a time-based archiving approach. Here’s what happens:

• Trigger: When archive mailbox is enabled for the user and the mailbox usage exceeds 90% of storage quota.

• Action: The Managed Folder Assistant moves the oldest items from the primary mailbox (IPM and Recoverable Items folders) to the archive until usage drops below 90%.

• SLA: The Managed Folder Assistant runs on a 7-day cycle per mailbox.

• Exceptions: Items tagged as “Never Move to Archive” remain untouched.

• Item Classes Excluded: Appointments, Tasks, Contacts, Distribution Lists, and certain file types.

• Report: Admins can track Auto-Archiving activity using PowerShell:

Auto-Archiving in Exchange Online helps keep mailboxes from running out of space. Traditional time-based policies like “Move to Archive after 2 years” only work after a specified time. So, mailbox can still fill up quickly because of large attachments, system notifications, or Copilot-generated meeting notes.

Note: Auto-Archiving is not the same as Auto-Expanding Archive. It does not automatically create additional archive storage.

Mailbox that reached size limit can cause:

• Mail flow interruptions
• Delayed or blocked email delivery
• Reduced productivity for users and admins

Auto-Archiving eliminates the need for admins to manually track mailbox storage while ensuring important historical data remains accessible. It integrates seamlessly with existing archiving policies and automatically returns to normal operation once mailbox usage falls below the defined limit.

Important: Auto-Archiving is a failsafe feature to protect mailbox health. It cannot be disabled and only runs when the mailbox archive is enabled for mailboxes in your Exchange Online environment.

Here’s a quick look at the key differences between traditional time-based archiving and the new threshold-based Auto-Archiving feature.

	Traditional Time-Based Archiving	Auto-Archiving (Threshold-Based)
Trigger	Moves items only after a fixed time period (e.g., 2 years).	Activates immediately when mailbox storage exceeds 90%.
Purpose	Helps manage mailbox size and organize older emails over time.	Focuses on preventing mailboxes from reaching full capacity.
Dependency	Requires retention tags and policies.	Requires only mailbox archive to be enabled (provisioned) with available storage.
Action	Organizes older emails but relies on fixed retention schedules, which may not prevent mailbox full errors.	Automatically prevents mailbox full errors by managing storage proactively.

Final Note

Thus, Auto-Archiving in Exchange Online will help admins to,

• Prevent mailbox quota issues and mail flow disruptions
• Keep older emails accessible and searchable in Outlook
• Work alongside existing retention policies
• Reduce manual mailbox management, giving more time for strategic tasks

Once Auto-Archiving rolls out fully, we’ll get to see how much it simplifies mailbox storage management. Hope this blog helps!