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As an M365 admin, one critical part of migrating from Office 365 E3 to E1 is handling the mailbox size change—from 100GB down to 50 GB. Users risk mailbox disruptions, data loss, and increased IT tickets if not managed correctly. Here’s the exact method I used successfully, with step-by-step details so you can replicate it without needing additional resources.

Why This Matters:

• Prevents sudden mailbox lockouts or email disruption for users.

• Avoids critical data loss by proactively managing mailbox storage.

• Reduces the number of user issues and IT helpdesk tickets after the migration.

Step-by-Step Guide to Reducing Mailbox Storage:

Step 1: Identify Oversized Mailboxes (>50GB)

Before making any changes, identify mailboxes that exceed the new 50GB limit.

Detailed Steps:

1. Open Windows PowerShell as Administrator.

2. Connect to Exchange Online (if you haven't already):

powershell

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Install-Module ExchangeOnlineManagement

Connect-ExchangeOnline

3. Run this command to list mailboxes larger than 50GB:

powershell

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Get-MailboxStatistics -ResultSize Unlimited | `

Where-Object {$_.TotalItemSize -gt 50GB} | `

Select DisplayName, @{Name="MailboxSizeGB"; Expression={$_.TotalItemSize.Value.ToGB()}} | `

Sort-Object -Property MailboxSizeGB -Descending

4. Export results for easy reference (optional):

powershell

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Get-MailboxStatistics -ResultSize Unlimited | `

Where-Object {$_.TotalItemSize -gt 50GB} | `

Select DisplayName, @{Name="MailboxSizeGB"; Expression={$_.TotalItemSize.Value.ToGB()}} | `

Sort-Object -Property MailboxSizeGB -Descending | `

Export-Csv -Path "C:\LargeMailboxes.csv" -NoTypeInformation

Result: You now have a clear report of which users will be impacted by the mailbox size reduction.

Step 2: Clearly Communicate Changes to Affected Users

Transparency reduces user frustration. Explain the upcoming storage change clearly and offer solutions upfront.

Detailed Steps:

• Draft and send a clear email to affected users with these key points:

  • Why mailbox size is changing: Cost-saving, efficiency, smoother performance.

  • Date when new limit will take effect: Clearly state timeline.

  • How users can archive emails: Provide direct, simple instructions.

Sample Email:

Subject: Important – Action Needed: New

Mailbox Storage Limit

Dear User,

As part of our migration to Office 365 E1,

your mailbox size limit will soon change

from 100GB to 50GB. This change takes

effect on [Insert Date].

To prevent disruptions, please reduce your

mailbox size by archiving older emails or

deleting unnecessary items.

Attached is a simple guide that shows you exactly how to:

1. Quickly remove unnecessary

attachments

2. Set up Outlook auto-archiving

If you need help, please contact IT support

immediately.

Thank you for your cooperation.

User Guide (One-page attachment):

• Removing large attachments:

  • Open Outlook, go to mailbox → click on "Search" → "Has Attachments" → sort by size, then delete unnecessary attachments.

• Using Outlook’s Auto-Archive:

  • Open Outlook → File → Options → Advanced → AutoArchive settings.

  • Enable AutoArchive and select emails older than a certain period (e.g., 12 months) to archive automatically.

    
    

• Step 3: Set Up an Automatic Retention (Archiving) Policy

Automated archiving ensures consistent

mailbox management without burdening

users.

Detailed Steps (Using Compliance Center):

1. Log into Microsoft Purview Compliance Portal.

2. Navigate:
   Compliance Portal → Data lifecycle management → Retention policies → "New retention policy."

3. Define policy clearly:

   • Name: "Auto-archive emails older than 2 years"

   • Select retention settings:

     • Retain items for: 2 years

     • Action after retention period: Move to archive (recommended)

4. Apply policy to:

   • All or specific users (based on your organization's needs)

5. Review and save your policy.

Result: Your policy is now active, ensuring older emails automatically archive, maintaining mailbox sizes under 50GB.

Step 4: Monitor Mailbox Sizes Proactively

Prevent future issues by periodically

checking mailbox sizes.

Detailed Steps:

1. Schedule a monthly calendar reminder

for yourself.

2. Each month, run this PowerShell

command again:

powershell

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Get-MailboxStatistics -ResultSize Unlimited | `

Where-Object {$_.TotalItemSize -gt 50GB} | `

Select DisplayName, @{Name="MailboxSizeGB"; Expression={$_.TotalItemSize.Value.ToGB()}} | `

Sort-Object -Property MailboxSizeGB -Descending

3. Email users who still exceed the limit a gentle reminder or offer support.

Sample follow-up email:

Subject: Friendly Reminder – Your Mailbox Size Exceeds 50GB

Dear User,

We noticed your mailbox size currently exceeds the 50GB limit. Please take a moment to archive older emails or delete

unnecessary attachments.

If you need assistance, our IT team is here to help!

Thank you!

Step 5: Handling Users with Extremely Large Mailboxes (80GB+)

In practice, some users require direct assistance.

• Schedule personal sessions to assist these users:

  • Offer live demos on archiving and organizing mailboxes.

  • Manually assist in setting up automatic archiving rules.

• Document their process clearly, so they feel supported and confident.

  
  

  Challenges I Experienced & How I Handled Them:

  
  

• User anxiety about losing important emails:
  Clearly emphasized “archiving” vs. “deleting,” reassuring users that nothing important gets lost.
  

• Resistance from users with very large mailboxes:
  Provided personal guidance, demonstrating the ease and benefits of proactive mailbox management.
  

• Concerns about productivity disruptions:
  Assured users by demonstrating practical steps and ensuring support availability throughout the process.

  
  

  
  

  Final Thoughts:

This detailed process worked effectively,

ensuring a smooth migration without data

loss or user disruption. Following these

clearly outlined steps, your migration will

also run smoothly—saving you significant

support time and preventing mailbox-

related issues down the road.