Explore technical insights and updates for Office 365 and Exchange Server. Priasoft empowers IT professionals with reliable solutions and expertise.

Tenant-to-Tenant Migration Checklist: What to Do Before You Move a Single Mailbox

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Office 365 Tenant to Tenant

A tenant to tenant migration — moving mailboxes, calendars, and content from one Microsoft 365 tenant to another — is most common in mergers, acquisitions, divestitures, and IT consolidations. It is also among the most disruptive migrations IT teams run, because the source and target are both live production systems throughout the process. This checklist covers the tenant to tenant migration preparation work that determines whether cutover goes smoothly.

The difference between a smooth cutover and a chaotic one is almost always what was done before the first mailbox moved. This checklist covers the preparation work that matters.

Before You Start: Decisions That Drive Everything Else

  • Define the scope precisely. Mailboxes only, or also Teams, SharePoint, OneDrive? Each adds complexity and tooling requirements. Scope creep mid-migration is a common cause of delays.
  • Set the target UPN and email domain. Will migrated users keep their source email address as a secondary alias, or move entirely to the target domain? This affects how mail routing is configured during coexistence.
  • Determine the coexistence period. Most tenant-to-tenant migrations involve a window where users on both tenants need to see each other’s calendars and send email. Plan for this explicitly — it requires GAL synchronization and free/busy coexistence, not just mailbox access.
  • Establish cutover criteria. What does “done” look like for each batch? Define success before you migrate a single mailbox.

Source Tenant Preparation

  • Inventory all mailboxes. Include shared mailboxes, resource mailboxes, room and equipment mailboxes, and distribution groups. These are commonly missed in initial scope estimates.
  • Audit licenses. Confirm the source tenant has the licenses that allow cross-tenant migration (Microsoft 365 cross-tenant migration requires specific license types). Confirm the target tenant has sufficient licenses for all migrating users before cutover.
  • Clean up before you migrate. Remove or disable stale accounts, consolidate redundant distribution groups, and clear orphaned delegates. Migrating a messy directory creates a messy directory in the target.
  • Document mail flow dependencies. Third-party applications, connectors, line-of-business systems that send mail via Exchange Online — every external dependency needs a post-migration plan.
  • Identify public folders. If your organization uses Exchange Online public folders, these require a separate migration track. They do not move with mailboxes. Plan for this separately or it will stall your project at 80%.

Target Tenant Preparation

  • Verify domain configuration. The target domain must be verified in Microsoft 365 before migration begins. Allow time for DNS propagation if the domain is new to the tenant.
  • Pre-create user accounts. Mail-enabled user objects need to exist in the target before mailboxes migrate. Automated user mapping based on UPN or email address saves significant manual effort at scale.
  • Configure coexistence. GAL synchronization ensures users on both tenants can see each other in the address book and check free/busy during the migration window. Without this, meeting scheduling breaks immediately after the first batch migrates.
  • Test mail flow end to end. Send test messages between source and target before committing any production mailboxes. Verify both directions, including from external senders.

During Migration

  • Migrate in batches, not all at once. Start with non-critical users or a pilot group. Validate before expanding to the next batch. Issues found in a pilot affect 20 users; the same issues found on cutover day affect everyone.
  • Run a delta sync before each cutover window. Mail received and sent after the initial sync needs to be captured before the user is moved. A delta sync immediately before cutover minimizes data gap.
  • Communicate to users before their mailbox moves. Users need to know their Outlook profile will need to be updated, their mobile devices will need to be reconfigured, and their calendar invites from the old domain may need to be resent.
  • Have Outlook profile updates ready. AutoDiscover often handles profile reconfiguration, but not always — especially in complex AD environments. Outlook Profile Update Manager automates this at scale for organizations where AutoDiscover is unreliable.

After Migration

  • Validate before decommissioning. Confirm all mailboxes are accessible, mail flow is working in both directions, and calendar data migrated correctly before removing the source tenant configuration.
  • Update all external systems. Any application, connector, or workflow that references the source tenant domain or user UPN needs to be updated. Make a list before you start.
  • Remove coexistence infrastructure. Once migration is complete and validated, remove GAL sync and other coexistence tooling. Leaving it running creates ongoing administrative overhead.

Tooling for Tenant-to-Tenant Migrations

Priasoft Express Migrator handles tenant-to-tenant Microsoft 365 migrations including mailboxes, calendars, contacts, and OneDrive content, with automated user mapping and batch processing. It is built for the M&A and divestiture scenarios where the source and target are both live production tenants throughout the migration window.

For coexistence during migration, Collaboration Suite (GalSync) keeps Global Address Lists synchronized and maintains free/busy visibility between tenants.

Free trials at priasoft.com/register-for-a-free-trial-download. For scoping a specific migration, our engineers take those calls.

What Is Super-ExMerge and Why Do IT Teams Still Need It in 2026?

Super-ExMerge (SXM)

Microsoft retired ExMerge in 2013. For IT administrators who worked with Exchange through the 2000s, it was the go-to utility for mailbox exports, PST creation, and selective data recovery. When Microsoft pulled it, many organizations simply stopped having a reliable tool for the job — which is exactly the gap Super-ExMerge for Exchange Online was built to fill.

The need did not go away. The tool did.

What ExMerge Did and Why It Still Matters

ExMerge extracted mailbox data from Exchange servers into PST files and merged PST data back into mailboxes. It supported filtering by date range, folder, sender, and subject — making it useful for eDiscovery, selective recovery, mailbox backup, and data migration.

In 2026, IT teams still need to do all of these things. Exchange Online does not eliminate the need for granular mailbox data operations — if anything, hybrid environments make them more complex. Legal holds, HR investigations, data recovery after accidental deletion, migration of specific folders between systems: these are everyday operations in any organization running Exchange or Exchange Online.

The difference is that most organizations are now doing them with tools that were not designed for the job, or with Microsoft’s built-in tooling that requires compliance admin access and significant setup overhead for simple tasks.

What Super-ExMerge Does

Super-ExMerge is Priasoft’s replacement for the retired ExMerge utility. It exports Exchange mailboxes to PST, imports PST data into mailboxes, and synchronizes mailbox data between Exchange servers. It supports filtering by date range, folder, sender, and subject — the same filtering that made ExMerge valuable — and extends that to Exchange 2010 through Exchange Online.

It is scriptable via command line, which means it fits into automated workflows and scheduled jobs rather than requiring manual intervention for routine operations.

Where IT Teams Use It in 2026

The use cases have not changed much since the ExMerge era. What has changed is the environment they operate in:

  • eDiscovery data collection — extracting specific mailbox content for legal or HR review without involving the compliance team for every request
  • Migration support — moving selected folders or date ranges rather than entire mailboxes, useful when a full migration is impractical or when post-migration cleanup is needed
  • Mailbox backup and archiving — creating PST snapshots of mailboxes before migrations, decommissions, or user offboarding
  • Data recovery — recovering specific items from a backup PST or from a secondary mailbox without a full restore
  • Cross-server synchronization — keeping mailbox data in sync between environments during staged migrations or coexistence periods

The Exchange Online Angle

Super-ExMerge works with Exchange Online, which is where most mailboxes now live. That matters because the operations IT teams need — selective export, targeted import, date-filtered extraction — are not well served by Microsoft’s native tooling for ad hoc operational use. The Microsoft Purview compliance portal is built for compliance workflows, not for the IT admin who needs to pull three months of a former employee’s sent items into a PST by end of day.

Super-ExMerge handles that directly, without requiring global admin access to the compliance portal or a formal eDiscovery case.

Getting Started

Super-ExMerge is available as a free trial at priasoft.com/register-for-a-free-trial-download. If you have a specific scenario you want to walk through first, our engineers take those calls.

Exchange 2019 Patches Just Ended. Here Is What IT Teams Need to Do This Month.

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Priasoft Migration Suite for Exchange (PMSE)

On April 14, 2026, Microsoft issued the last security patch it will ever release for Exchange Server 2016 and Exchange Server 2019. If your organization is still running either version, you are now on a server that cannot be secured — only replaced.

This is not a grace period. Extended Security Updates are over. The patch window is closed permanently.

What End of Extended Support Actually Means

Microsoft’s support lifecycle has two phases: mainstream support (new features, bug fixes, security patches) and extended support (security patches only). Exchange 2019 mainstream support ended January 9, 2024. Extended support ended April 14, 2026.

After extended support ends, Microsoft stops publishing security updates. Researchers continue finding vulnerabilities. Attackers continue exploiting them. Your server does not get fixed.

For a system that handles authentication, inbound SMTP, and in many organizations, calendar and contacts for the entire company, that is a meaningful exposure.

The Enforcement System That Is Already Running

Microsoft has a live transport enforcement system in Exchange Online that identifies “persistently vulnerable” on-premises servers and throttles, then blocks, their ability to send mail to Exchange Online. The system has been active since 2025 and covers any server that is end-of-life or significantly behind on patches.

With Exchange 2019 now permanently unpatched, servers running it will eventually trigger this system. The timeline depends on when new vulnerabilities are discovered and when Microsoft’s scanner flags your specific build. But the direction is clear: staying on Exchange 2019 is not a stable long-term posture.

Once flagged, you have 90 days to remediate before mail flow is blocked. Remediation at that point means migration — there are no more patches to apply.

What IT Teams Should Do This Month

If you are running Exchange 2016 or 2019, here are the immediate steps:

  • Confirm your current build. Run Get-ExchangeDiagnosticInfo or check the Exchange Admin Center. Know your exact version and CU level.
  • Apply the last available CU and SU now. Exchange 2019 CU15 with the April 2025 Security Update is the final patch set. If you have not applied it, do so immediately — it is the last protection you will get.
  • Audit your hybrid configuration. Understand how your on-premises server connects to Exchange Online. Note any applications using EWS, SMTP AUTH, or Basic authentication — each of these has a 2026 deprecation deadline.
  • Identify what is keeping you on-premises. For most organizations still on Exchange 2019, the answer is public folders. Audit them with Public Folder Analyzer — free, runs in under an hour, gives you the scope you need to plan migration.
  • Start planning the migration now. Not next quarter. The 90-day enforcement clock starts the moment Microsoft flags your server. You do not control when that happens.

The Other 2026 Deadlines You Need to Know

Exchange 2019 reaching end of support is the headline, but it lands alongside several other Microsoft deprecations that affect hybrid environments:

  • SMTP AUTH Basic auth — retired from Exchange Online in March 2026
  • Legacy ActiveSync clients below EAS 16.1 — blocked from Exchange Online as of March 1, 2026
  • Exchange Web Services (EWS) in Exchange Online — phased retirement begins October 1, 2026; full shutdown April 2027

If your environment touches any of these — and most hybrid environments do — the window to act without disruption is narrowing.

The Path Forward

For most organizations, migration to Exchange Online is the right answer. For those with regulatory or operational reasons to stay on-premises, Exchange Server Subscription Edition (Exchange SE) is the current supported version.

Either way, the migration off Exchange 2019 is not optional — it is a matter of when, and whether you control the timing or Microsoft’s enforcement system does.

Priasoft’s Migration Suite for Exchange handles Exchange-to-Exchange and Exchange-to-Office 365 migrations including cross-forest, inter-org, and hybrid scenarios. Speak with an engineer about your environment, or start with a free trial download.

The Part of Your Exchange Migration You Put Off Is Now a Security Liability

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Priasoft Public Folder Migration Manager for Exchange and Office 365

The Exchange Server Nobody Wants to Touch

For many organizations, the move to Microsoft 365 is largely complete. Mailbox migration is done, users are operating in Exchange Online, and day-to-day administration has shifted to the cloud. Yet public folder migration is often the piece that was deferred — and the Exchange server it left behind is now a growing liability.

Yet one component often remains on-premises: public folders.

Public folder migration gets deferred during the initial project due to complexity, competing priorities, or unexpected errors mid-migration. What begins as a short delay often extends into months or years, leaving an Exchange server running solely to support legacy folder data.

That server is no longer a temporary dependency. And as of right now, it has a clock running against it.

Microsoft Is Already Blocking Email From Unpatched Servers

This is not a future risk. Exchange Online’s transport enforcement system is live and actively flagging on-premises servers it considers “persistently vulnerable.” The process works in three stages: reporting, throttling, then a full block on inbound email. Once your server is flagged, you have a 90-day remediation window. After that, mail stops flowing.

Any server running an end-of-life Exchange version qualifies — Exchange 2013 and earlier are already in scope. Exchange 2016 and 2019 servers that are significantly behind on security updates are next.

Which brings us to April 14, 2026.

The Patch Window Just Closed — Permanently

Exchange 2016 and Exchange 2019 Extended Security Updates ended on April 14, 2026 — eight days ago. Microsoft will not issue another security patch for either version. Every vulnerability discovered from this point forward goes unaddressed on your server.

This is the trigger that changes the calculation. A server that was “behind on patches” is manageable. A server that cannot be patched will eventually trip Microsoft’s enforcement system with no remediation path except migration.

The 90-day clock, once it starts, ends with your on-premises Exchange server losing the ability to deliver mail to Exchange Online. For organizations in a hybrid configuration — which describes almost every organization that still has public folders on-premises — that means disrupted mail flow for real users.

More Deadlines Hitting in 2026

The mail flow block is the most immediate risk, but it is not the only one:

  • SMTP AUTH Basic authentication — retired March 2026. Exchange Online permanently removed Basic auth for client submission. Hybrid configurations using it for SMTP relay stopped working last month.
  • Legacy ActiveSync clients — blocked March 1, 2026. Exchange Online now requires EAS version 16.1 or higher. Older mobile clients lose access to Exchange Online mailboxes.
  • Exchange Web Services (EWS) — retirement begins October 1, 2026. EWS access to Exchange Online is being disabled. Hybrid environments where applications access cloud mailboxes via EWS will break. Full shutdown completes April 2027.

Each of these affects hybrid environments differently. But every one of them is a reason the on-premises Exchange server that exists only for public folders is now actively working against you.

Why Public Folder Migrations Stall

Public folder migrations fail or stall for predictable reasons — and they are rarely the same reasons mailbox migrations fail:

  • Mail-enabled public folders require preparation steps that native tooling handles poorly
  • Large or deeply nested hierarchies hit quota and performance limits mid-migration
  • Complex permission models — nested groups, anonymous access, legacy role assignments — do not transfer cleanly
  • Native tooling provides poor diagnostics when errors occur, leaving migrations stopped with no clear path forward

Most organizations that stalled got to 60–70% and stopped. That is a recoverable position — but only if you act before Microsoft’s enforcement system acts for you.

Start with Accurate Visibility

Before resuming or reattempting a migration, you need a complete inventory: total size and item count, active versus inactive folders, permission structures, mail-enabled status, and any non-standard content types. Without that baseline, you will hit the same issues that caused the stall.

Priasoft’s Public Folder Analyzer runs against Exchange 2010 through 2019 and produces a complete inventory — size, item count, permissions, mail-enablement, cleanup candidates — in about an hour. It exports to CSV and it’s free. Run this before scoping anything else.

What Completing the Migration Involves

Public folder migration is distinct from mailbox migration and requires a structured approach:

  1. Discovery and inventory — establish a complete view before anything moves
  2. Cleanup — remove stale folders and resolve broken permissions on the source
  3. Hierarchy and content synchronization — migrate in stages with validation between each phase
  4. Mail-enablement alignment — configure mail-enabled folders in the target before cutover
  5. Delta synchronization and cutover — capture changes and finalize the transition
  6. Decommission — remove the source server and close out the project

Many stalled migrations can be resumed rather than restarted, provided the current state is understood.

When Native Tools Are Not Enough

In environments with cross-forest migrations, tenant-to-tenant moves, multi-server public folder deployments, or complex permission hierarchies, Public Folder Migration Manager provides the control, visibility, and permission fidelity that standard tooling lacks. If your migration stalled at 60–70% and you are not sure why, this is where to look.

For a complete technical walkthrough of every phase, the complete public folder migration guide covers it end to end. Free to access.

The Clock Is Running

If Exchange remains on-premises only because of public folders, the migration is not complete — it is paused. With Extended Security Updates now closed, Microsoft’s enforcement system already active, and three additional deprecation deadlines hitting in 2026, the cost of continuing to defer is no longer theoretical.

Speak with a Priasoft engineer about your specific environment. Bring what you know — Exchange version, folder count, where the migration stalled — and we will tell you what the path forward looks like.

Free trial downloads at priasoft.com/register-for-a-free-trial-download.

The Hidden Dangers of Public Folders: Shocking Statistics Every IT Manager Needs to Know

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Exchange public folder security risks for IT managers

Discover the hidden dangers of public folders and the alarming state of their security. Learn how to mitigate risks.

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Tenant to Tenant Migration (O365) – A Comprehensive Guide

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office 365 tenant to tenant migration in a fast-paced business environment

Last reviewed: April 2026 — checked against current Microsoft product lifecycle and Exchange Online enforcement timelines.

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How to Migrate Public Folders to Office 365 – The Complete Guide

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Public Folder Migration Guide

Learn how to migrate public folders to Office 365 with our free comprehensive guide. Gain insights from two decades of expertise in Microsoft Exchange Public Folder Migration.

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Addressing Pain Points in Restricted Access Migrations to Office 365  – Optimizing Mailbox Data Migration and Public Folder Migrations

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Restricted Access Migrations to Office 365
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How To Tackle eDiscovery, Compliance, and Outlook PST Headaches with ease

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How To Tackle eDiscovery, Compliance, and Outlook PST

In the fast-paced world of legal proceedings, where time is critical and precision is a must, the challenges of eDiscovery have never been more important. Legal professionals often find themselves overwhelmed by mountains of email data, desperately searching for that crucial piece of evidence that could turn the tide in their favor.

Enter Super-ExMerge, your comprehensive solution for simplifying eDiscovery. In this excerpt, we explore how Super-ExMerge can transform your eDiscovery process, saving you time, preserving data integrity, and keeping your legal team ahead of the game.

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Simplifying the Migration of Large Exchange Archive Mailboxes to the Cloud: Overcoming Throttling Challenges

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Office 365 Archive Migration
Last reviewed: April 2026 — checked against current Microsoft product lifecycle and Exchange Online enforcement timelines.

Migrating large Exchange archive mailboxes to the cloud poses a significant challenge due to throttling issues that can hinder the smooth transfer of data. Throttling, a mechanism designed to maintain system performance and prevent overload, often occurs during migrations of large volumes of data, such as Exchange archive mailboxes. Microsoft’s article on auto-expanding archiving highlights the importance of being aware of throttling limitations when transferring large amounts of data to the cloud. Throttling can significantly impact migration efficiency, leading to extended time-frames, failed migrations, and frustration among end users.

In this blog post, we will address the throttling problem organizations face during such migrations and present a solution that simplifies the migration process by mitigating throttling concerns. Our approach combines industry best practices with innovative strategies to ensure a seamless transition of large archive mailboxes to the cloud.

To overcome throttling challenges, we recommend leveraging specialized migration tools designed to handle large data volumes efficiently. One such solution is Priasoft Super ExMerge. This powerful tool incorporates features specifically tailored to address the throttling issues encountered during the migration of large Exchange archive mailboxes. Let’s explore how Super ExMerge can help simplify your migration process and mitigate throttling concerns:

  1. Dynamic Multi-Threading: Super ExMerge intelligently manages threads during the migration, ensuring the maximum utilization without overwhelming the system. This feature optimizes performance, minimizing the impact of throttling and expediting the migration process.
  2. Multi-Process Capability: Leveraging the power of multi-process architecture, Super ExMerge can handle multiple migration tasks concurrently. By utilizing available CPU and RAM resources efficiently, it significantly improves throughput and mitigates throttling-related slowdowns.
  3. Efficient Data Transfer: Super ExMerge only copies new and changed data, thanks to its full fidelity synchronization tracking. This smart approach eliminates unnecessary data transfer, reducing the overall volume of data to be migrated. As a result, throttling concerns are alleviated, and the migration process becomes more streamlined.
  4. Granular Control and Folder Exclusion: Administrators have granular control over the migration process, enabling them to select specific folders or subtrees for synchronization. Additionally, the ability to exclude certain folders minimizes the migration load, addressing potential throttling issues effectively.
  5. Authentication Flexibility: Super ExMerge allows each migration task to authenticate with different accounts, distributing the migration load across multiple credentials. This capability prevents throttling limits imposed by Microsoft Exchange, ensuring a smooth and uninterrupted migration.
  6. Comprehensive Reporting and Logging: Super ExMerge provides detailed reports and logs, offering valuable insights into the migration progress. Administrators can closely monitor the process, identify potential bottlenecks, and take necessary actions to mitigate throttling challenges promptly.

By adopting our alternative solution and utilizing specialized migration tools like Priasoft Super ExMerge, organizations can streamline the migration process and mitigate throttling concerns. This approach ensures a seamless transition to the cloud, minimizing downtime, maximizing data integrity, and enabling your organization to fully embrace the advantages of a cloud-based infrastructure.

Contact our Exchange Engineers today to learn more about our solution and how we can assist you in simplifying the migration of large Exchange archive mailboxes while overcoming throttling challenges. Say goodbye to the frustrations of throttling and embrace a smoother migration journey with Super ExMerge.

Speak with an Engineer