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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.

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 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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Unlock Your Microsoft 365 Migration Success: FastTrack and Priasoft – The Winning Duo

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Priasoft_FastTrack_FI

Unlock Your Microsoft 365 Migration Success: FastTrack and Priasoft – The Winning Duo 🚀

Embarking on a Microsoft 365 migration journey? Discover how Microsoft FastTrack provides a solid start and why combining it with Priasoft’s expertise can maximize your efficiency, security, and ROI. Say goodbye to migration headaches and hello to a seamless transition! Learn more in our latest article.

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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.

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    Urgent Alert: Microsoft to Block Emails from Unsupported Exchange Servers – Risks and Solutions

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

    Microsoft has issued an alert regarding the risks of unaddressed Exchange Server patching and security issues that can impact email delivery speed and potentially lead to email throttling if not addressed. If these issues are not resolved within 30 days, emails may be blocked, causing severe disruptions in communication with customers and partners. Read on for vital information on how to mitigate these risks and safeguard your email operations.

    Microsoft’s recent announcement highlights that unresolved issues on unsupported or outdated On-premises Exchange servers can result in security and other potential risks. Customer should address these issues as quickly as possible to avoid Microsoft taking action in the form of email throttling, causing significant delays in email delivery. This can lead to missed opportunities, frustrated customers, and damaged business reputation. However, the urgency goes beyond that. If these issues are not addressed within 30 days, Microsoft may take further action and block emails from these servers, resulting in disruptions in communication.

    We’ve said many times that it is critical for customers to protect their Exchange servers by staying current with updates and by taking other actions to further strengthen the security of their environment, Microsoft

    To avoid such consequences, it’s crucial to take immediate action. Here are critical steps to protect your business:

    1. Address Known Issues: Conduct a comprehensive review of your Exchange servers for any known issues or errors and resolve them promptly. This may involve applying relevant patches, updates, or configuration changes to eliminate potential bottlenecks in email delivery.
    2. Keep Exchange Servers Up to Date: Ensure your Exchange servers are running the latest supported version and have all the necessary security updates applied. Regularly monitor for new updates and apply them promptly to maintain a secure and reliable email communication environment.
    3. Monitor Email Delivery Speed: Keep a close eye on the speed of email delivery from your Exchange servers to Exchange Online. If you notice significant delays, investigate and resolve any potential throttling issues promptly to prevent prolonged disruptions.
    4. Seek Expert Assistance: If you’re unsure about the status of your Exchange servers or need help with addressing issues and monitoring email delivery speed, consider seeking assistance from IT professionals or Microsoft support for timely resolution.
    5. Leverage the new Mail Flow Report: Microsoft will release and new report in the Admin Center that will provide information about Exchange servers that are unsupported in their environment.

    The risks of unaddressed issues on your on-premises Exchange servers are imminent, with the potential for email throttling and complete blockage within 30 days. Don’t wait for disruptions to occur – take immediate and proactive steps to address known issues, keep your Exchange servers up to date, monitor email delivery speed, and seek expert assistance if needed.

    In conclusion, Microsoft’s urgent alert underscores the need for immediate action to mitigate risks and ensure uninterrupted email operations. Take the necessary steps to address issues, keep your Exchange servers up to date, monitor email delivery speed, and seek expert assistance if required. By taking proactive action now, you can prevent email throttling, potential blockage, and maintain seamless email communication with your customers and partners. Act urgently to safeguard your business from the risks of unaddressed issues on your Exchange servers.

    To learn more see Throttling and Blocking Email from Persistently Vulnerable Exchange Servers to Exchange Online.