If your Office 365 mailbox primary and archive storage have reached their limits, it can interrupt email functionality, preventing the receipt of new messages and causing workflow disruptions. Below, we’ll explore actionable steps you can take to resolve this issue and ensure uninterrupted email services.

Why Does This Happen?

1. Mailbox Limits: Depending on your Microsoft 365 license, the storage limits for primary and archive mailboxes vary:
   • Microsoft 365 Business Standard or Exchange Online Plan 1: 50 GB primary + 50 GB archive.
   • Microsoft 365 E3/E5 or Exchange Online Plan 2: 100 GB primary + unlimited archive (with auto-expansion enabled).
2. Storage Overload: Heavy email usage with large attachments, excessive retention policies, and infrequent cleanup can cause mailboxes to hit their limits.

Office 365 mailboxes, including both primary and archive mailboxes, are essential for managing emails, attachments, and organizational communications. When these mailboxes reach their storage limits, it can disrupt workflows, as users can no longer send or receive emails. As an Office 365 administrator, there are multiple strategies to resolve this issue and ensure continued productivity.

Below are actionable steps you can take to resolve and prevent mailbox storage issues in Office 365.

1. Understand the Mailbox Storage Limits

Before taking corrective actions, it’s important to know the storage limits for primary and archive mailboxes, as they vary depending on the user’s license.

• Microsoft 365 Business Plans (Standard, Basic, etc.):
  • Primary Mailbox: 50 GB
  • Archive Mailbox: 50 GB
• Microsoft 365 Enterprise Plans (E3, E5, etc.):
  • Primary Mailbox: 100 GB
  • Archive Mailbox: Unlimited (with Auto-Expanding Archive enabled)

If the mailbox is nearing these limits, administrators have several options to manage and reduce mailbox storage.

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2. Enable Auto-Expanding Archives

If users are on a plan that supports auto-expanding archives (e.g., Microsoft 365 E3/E5), this feature can automatically provide additional storage for the archive mailbox beyond the default 50 GB.

Steps to Enable Auto-Expanding Archive:

1. Open the Microsoft Purview Compliance Portal (compliance.microsoft.com).
2. Navigate to Information Governance → Archive.
3. Select the user’s mailbox and enable Auto-Expansion for the archive.

Impact:

• Archive mailboxes can grow incrementally up to 1.5 TB, ensuring that storage constraints are avoided.

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3. Clear Mailbox Space

Freeing up storage in the primary and archive mailboxes is often the quickest solution.

a. Encourage Users to Clean Up Their Mailboxes

• Delete Unnecessary Emails:
  • Sort by size in Outlook or Outlook on the Web (OWA) and delete large emails or attachments.
• Empty Deleted Items and Junk Folders:
  • Regularly clean up the Deleted Items and Junk Mail folders.
• Use Conversation Cleanup:
  • Use the Clean Up Conversation feature in Outlook to remove redundant emails.

b. Use Retention Policies to Automate Cleanup

As an administrator, you can create retention policies to automatically delete older, unnecessary emails:

1. Go to the Microsoft Purview Compliance Portal.
2. Navigate to Data Lifecycle Management → Retention Policies.
3. Create a new policy to delete or archive emails older than a specific time period.

Impact:

• Retention policies reduce mailbox size by archiving or deleting emails based on defined criteria.

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4. Migrate Data to External Storage

If the mailbox contains valuable data that cannot be deleted, consider migrating it to external storage.

Options:

• Export Mailbox to PST:
  • Use Outlook to export emails to a PST file:
    • Go to File → Open & Export → Import/Export → Export to a File → Outlook Data File (.pst).
  • Store the PST file on the user’s device or a shared network location.
• Move Emails to SharePoint or OneDrive:
  • Save emails or attachments to SharePoint or OneDrive for easier access and reduced mailbox size.

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5. Assign a Larger Mailbox with License Upgrades

If the user’s work requires storing large volumes of emails and attachments, upgrading their license is a long-term solution.

Upgrade to Enterprise Licenses:

• Enterprise E3 or E5 Plans:
  • Primary Mailbox: 100 GB.
  • Archive Mailbox: Unlimited (with Auto-Expanding Archives).

How to Upgrade:

1. Go to the Microsoft 365 Admin Center.
2. Navigate to Billing → Licenses.
3. Assign the upgraded license to the user.

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6. Investigate and Reduce Attachments

Large attachments are often the cause of excessive mailbox usage. As an administrator, you can:

• Use mail flow rules to prevent or restrict large attachments.
• Encourage users to store large files in SharePoint, OneDrive, or Teams and share links instead of attachments.

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7. Monitor and Set Alerts for Storage Usage

Prevent future storage issues by monitoring mailbox usage and setting alerts when limits are approached.

Steps:

1. Use PowerShell to Monitor Mailbox Size:powershellCopyEditGet-MailboxStatistics -Identity user@domain.com | Select DisplayName, TotalItemSize, ItemCount
2. Set Mailbox Size Alerts:
   • Create alerts to notify admins or users when a mailbox reaches 90% of its storage capacity.

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8. Communicate with Users

Let users know when their mailbox is nearing capacity and provide them with guidance on managing storage:

• Share best practices for email cleanup.
• Recommend moving older emails to archives or external storage.