How to Migrate Archive Mailbox Over 100GB to Exchange Online Effortlessly?

How to migrate archive mailbox of over 100GB to Exchange Online? Is a question that every admin fears. During a business lifecycle, it is natural that user mailbox sizes balloon out of proportion.

As in an on-premise system, there is no limit on how much external storage you can attach to your server these ever-expanding mailboxes may grow to 100GB or beyond.

Whereas M365 cloud systems have strict limits on the amount of data a user mailbox can contain.

So when organizations reorient themselves and want to transition to the cloud these large mailboxes become the biggest hurdle.

However, there is no need to worry with a solid plan we will explain to you what Microsoft’s own best practices are. Uncover the technical speed breakers, and, importantly, give instructions on how to use a tool that can perform a large archive migration.

Let us start by understanding the scale of the challenge on our hands.

See Microsoft 365’s Boundaries Before Migrating Archive Mailbox Over 100GB to Exchange Online

Solutions would only make sense if our source data is in the correct format. There are some key thresholds that admins must be familiar with while moving massive archives to Microsoft 365.

1.5 TB Total User Data Limit: Every licensed user mailbox comes under a hard limit of 1.5 Terabytes. This total value includes the primary mailbox, main archive, and any auto-expanding archive space. No matter what M365 plan you subscribe to it is not possible to go over this limit. Admins have to distribute it by splitting and placing data in other licensed or shared mailboxes.

240GB Onboarding Archive Limit: This is another storage limit preventing the free flow of data from on-premise to the cloud. The official Microsoft states that “Onboarding an on-premises archive of more than 240 GB to Microsoft 365 is currently not supported.” This means that if any account has enough emails that exceed this limit you need to take a diffrent route.

Deleted Item Retention Timeline: Microsoft 365 has a default retention policy for deleted items fixed at 14 days. Although its max value can be configurable to 30 days. No such strict limits are there in on-premises setups. So if your organization’s source exceeds this cloud limit, and is not on hold, it is at risk of permanent removal post-migration.

Cloud Archives & Auto-expanding Archive: Before you try to move large archives > 100 GB ensure that the cloud archives with complementary auto expansion is pre-provisioned for every account.

Understanding these limitations upfront sets realistic expectations and guides your migration strategy. Ignoring them can lead to significant roadblocks and data loss. Now let’s look at what Microsoft suggests you do.

Use Retention Policies for Pre-Migration Data Management

Proactive Data Offload:

Make room by using custom Retention policies to move all old and low-priority data from the primary mailbox to the M365 cloud archive. This is done to keep the primary mailbox size under 100 GB which results in faster migration.

Depending on your on-premise data distribution the steps vary a bit. Let us address them in a scenario-wise fashion.

• Scenario 1: Users Without On-Premises Archive (Large Primary Mailbox)
  • For Mailboxes 100GB–240GB:
    • Make a retention policy to deposit recoverable items to the M365 cloud archive.
    • Check the mailbox size; if it is < 100 GB, migrate via Cutover, Staged, Hybrid, etc.
    • If the size is > 100 GB, add more retention policies (e.g., move emails older than 2 years or calendar items older than 3 years) until the size is reduced.
  • For Mailboxes >240GB:
    • Discard redundant data or apply hyper-aggressive retention policies till the primary mailbox size is < 100 GB threshold.
• Scenario 2: Users With On-Premises Archive
  • When Archive is <240GB and Primary >100GB:
    • Perform an archive first migration only after ensuring that the auto-expanding archive is enabled.
    • Once archive migration is complete, apply retention policies on the primary mailbox to remove recoverable/older items until it is well under 100GB.
    • Continue with primary mailbox migration.
• Additional Recommendations:
  • Increase the MoveToArchiveTotalCountLimit (from 2000 to 10000) on on-premises Exchange servers and restart the MSExchangeAssistants service to boost processing speed.
  • For critical, priority, or VIP mailboxes, manually trigger the ELC process more frequently via the Start-ManagedFolderAssistant cmdlet (with caution to avoid server overload).
  • Adjust the ELC work cycle (e.g., from daily to every 8 hours) while maintaining the balance between speed and server load.
  • Always keep retention policies within compliance and legal requirements.

Now that we know how to approach migration in various scenarios let us see what native methods Microsoft has for us.

Manual M365 Migration of Archive Mailbox Over 100GB to Exchange Online

Although setting up the retention policies and enabling the auto archive expansion allows the migration of mailboxes that are well over 100GB they don’t do the actual migration.

For that, there are a set of techniques described below. Moreover, with Exchange Server 2016 and 2019 set to lose official support, now is the critical time to evaluate and execute your migration strategy.

• Cutover Migration: Used for moving entire organizations in one go, this method migrates primary mailboxes after retention policies pre-stage data to cloud archives, though it’s less optimal for very large archives.
• Staged Migration: Ideal for gradual transitions, staged migration also handles primary mailboxes post-archive pre-staging, offering flexibility for larger environments.
• Hybrid Deployment: This long-term co-existence model is essential for phased migrations and ongoing Exchange management.  Moreover, it uses retention policies to manage large archives and MRSProxy-based moves for the final migration of primary mailboxes.
• IMAP Migration: Generally slower and less feature-rich, this method is not recommended for large Exchange archives due to throttling issues; it’s only viable for small archives or non-Exchange systems.

Monitoring and Troubleshooting Large Archive Migrations >100 GB to Exchange Online Cloud

Monitoring Via:

• PowerShell Cmdlets: Use Get-MoveRequestStatistics to monitor move request status, progress, and throttling issues.
• Tool Dashboard: The software provides real-time dashboards and reports for tracking migration progress and post-migration auditing.

Troubleshooting Common Issues:

• Slow Migration: Check network performance, source load, and Microsoft 365 throttling via built-in dashboards.
• Stalls or Hangs: Look for hardware throttling or individual mailbox issues; review error logs for clues.

Conclusion

In this guide on how to migrate archive mailbox over 100GB to the Exchange online platform, we gave all the challenges that admins may encounter.

However, through careful application of retention policies and enabling the auto archive feature admins can perform the migration.

Moreover, instead of relying on native methods, it is better to use a professional tool that lacks many of the built-in roadblocks and allows for a smooth on-premise-to-cloud transition.