Demystifying Exchange Online Mailbox Quotas

Mailbox quotas govern storage limits in Exchange Online

Mailbox quotas in Exchange Online enforce storage limits across different mailbox components. Understanding these quotas prevents user disruptions and helps administrators manage resources effectively.

Core Mailbox Quota Types

1. Primary Mailbox Quota (ProhibitSendReceiveQuota)

• Function: Blocks sending/receiving emails when exceeded
• Default: 100GB (varies by license)
• Check: Get-Mailbox user | FL ProhibitSendReceiveQuota
• Usage: Get-MailboxStatistics user | FL TotalItemSize

2. Archive Mailbox Quota (ArchiveQuota)

• Function: Prevents adding content to specific archive shard
• Default: 100GB (auto-expanding archives use multiple shards)
• Key Insight: Auto-expanding provides 1.5TB total but each shard caps at 100GB
• Check: Get-Mailbox user | FL ArchiveQuota

3. Recoverable Items Quota (RecoverableItemsQuota)

• Function: Blocks purging items when full
• Default: 30GB (non-hold users) or 100GB (hold users)
• Check: Get-Mailbox user | FL RecoverableItemsQuota
• Usage: Get-MailboxStatistics user -Archive | FL TotalDeletedItemSize

Example quota configuration in PowerShell

Checking Storage Usage

Component	Command	Key Parameter
Primary Mailbox	Get-MailboxStatistics user	TotalItemSize
Archive Mailbox	Get-MailboxStatistics user -Archive	TotalItemSize
Recoverable Items	Get-MailboxStatistics user	TotalDeletedItemSize

Folder-level analysis:
Get-MailboxFolderStatistics user -FolderScope RecoverableItems | FT Name, FolderSize

Critical Scenarios & Solutions

Scenario 1: Migration Failures

• Problem: Cross-tenant migrations failing due to recoverable items limits
• Solution: For Mail Users:
  Set-MailUser user -EnableLitigationHoldForMigration
  Increases recoverable quota to 100GB

Scenario 2: Full Primary Mailbox

• Solutions:
  1. User cleanup (delete unnecessary items)
  2. Enable archive mailbox
  3. Upgrade to Exchange Online Plan 2 (100GB limit)

Scenario 3: Full Archive Mailbox

• Solution: Enable auto-expanding archive
  Set-Mailbox user -AutoExpandingArchiveEnabled $true
  Creates auxiliary archives when main archive fills

Auto-expanding archive architecture

Pro Tips

1. Hold Status Increases Quotas:
   Litigation hold boosts recoverable items quota to 100GB
2. Auto-Expanding Side Effects:
   Disables mailbox restoration/offboarding
3. Monitor: Use M365 mailbox usage reports

Best Practices

• Check quotas quarterly via Get-Mailbox
• Enable auto-expanding archive proactively
• Address warnings at 90% capacity (IssueWarningQuota)

Additional Resources

• Recoverable Items Quota Increase
• Auto-Expanding Archiving Guide

Documentation updated: January 2026 | Version 2.0