Cross-tenant mailbox migration: A step-by-step guide

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Understand the basics of Microsoft 365 tenant-to-tenant mailbox migration, how to plan and execute a seamless move, and how to keep your end users in mind during the process.

Your organization is growing, or maybe there’s been a merger. Suddenly, your team has the new mission to migrate Exchange Online mailboxes from one tenant to another in Microsoft 365.

As an IT admin, you’d love nothing more than a flawlessly executed mailbox migration process because you know that a seamless migration is vital for keeping your data intact and guaranteeing that your end users’ workflow runs like clockwork uninterrupted. Yet, achieving that can be daunting. Ideally, every element of a cross-tenant mailbox migration would unfold like a perfectly orchestrated dance, with each step flowing effortlessly into the next.

We’ve got your back with this straightforward guide. We’ll break down the process into easy-to-follow steps, making sure you’re prepped, planned, and ready for a smooth cross-tenant mailbox migration. And, you’ll learn what tools and resources are available to complete the migration process. Let’s go!

What is a cross-tenant mailbox migration? 

A cross-tenant migration is when you move data and services between different Microsoft 365 tenants. A cross-tenant mailbox migration would specifically involve transferring email data associated with your mailbox source tenant to the target tenant.  

It’s something you typically encounter in various scenarios from mergers and acquisitions to organizational restructuring. The goal is to transfer those resources safely and smoothly without major disruptions.

What content can I migrate? 

During this migration, you’ll move things users can directly see and use in their mailbox. Here’s an overview of what data you would typically migrate:  

  • Email data, including folders, items, drafts, and deleted items 
  • Calendar events, appointments, recurring and group events, and meetings, including event attachments and privacy settings 
  • Contacts, including names, email addresses, phone numbers, and organizational details 
  • Tasks, reminders, notes, and memos stored in the mailbox 
  • Distributed lists 
  • Shared mailboxes 
  • Any server-side rules (the automated actions that you’ve set up for certain scenarios, like out-of-office replies or forwarding incoming messages)   

When and why perform a cross-tenant mailbox migration?

You may face a range of scenarios where your organization would need to shift your mailboxes to a new and/or different setup.   

A shake-up at your organization 

Strategic restructuring is commonplace in a modern workplace, whether to trim costs, tighten security, or tackle other critical business goals. For example, your organization may have merged with or acquired another company, so you might need to consolidate different workloads. Or perhaps you’re undergoing some internal reorganization, like centralizing operations, which requires consolidating resources, or a department split, which requires a new organizational structure. 

To complete these projects, your organization will likely need to overhaul some or all of its IT infrastructure. And as part of that work, you would need to migrate one of the most integral elements underpinning your day-to-day operations: your email system!  

A broader desire to boost productivity 

Finally, you might simply be looking to undergo a tenant migration to make your operations more efficient. If so, shifting your email resources to a new environment could be exactly what the doctor ordered to optimize your IT resources and create a more standardized, cost-effective user experience. 

How to plan for a migration 

Embarking on a cross-tenant migration will take some meticulous planning. Here are the most critical aspects to consider and steps to take if you want to lay the groundwork for a successful transition. 

Step 1: Know what you’re migrating 

Before you can plan for how you’ll migrate your data, you need to first understand the data you’ll be moving. You wouldn’t try to bake a cake without knowing all the ingredients first, right? 

Inventory your current email environment, including the number of mailboxes, their sizes, and any special requirements or constraints. That will help you allocate sufficient resources and tailor your strategy to your organization’s needs.  

Pro tip: The Exchange Admin Center is a good place to source that information! 

As part of this asset audit, you’ll want to look at your: 

  • Email data, including your inboxes, contacts, calendars, tasks, and notes.  
  • User accounts, including your active/inactive users, shared mailboxes, and distribution lists. 
  • Public folders 
  • DNS records 
  • Retention policies (archiving settings, legal holds) 
  • Dependencies/integrations (third-party applications, email forwarding rules, or mailbox permissions) 

Step 2: Choose your migration method 

Next, it’s time to figure out how you’ll migrate all your mailbox data. Selecting the most appropriate technique will ensure that your end users are happy with the experience. 

The method you choose will depend on your current email system, your desired timeline for your tenant migration project, the number of mailboxes you’ll be moving, and other factors. Here are the three options: 

  • Cutover: This method means that you will migrate all of the email content in one go. (This is appropriate if you’re moving less than 2000 mailboxes and running Exchange 2003 or 2007, although Microsoft recommends doing 150 or fewer at a time for the sake of time and simplicity.) 
  • Staged: This method involves migrating your mailbox content over time. (Microsoft recommends this option for moving more than 2000 mailboxes while running Exchange 2003 or 2007.) 
  • Hybrid: With this method, an organization maintains both the source and target environments simultaneously during a cross-over period, and it can gradually migrate its mailboxes at its own pace.  

Step 3: Prepare your source environment 

To experience a smooth, secure, and efficient mailbox migration process, you’ll need to adequately prepare your source environment before the transition. This will help prevent any hiccups due to technical errors and data loss, and help your users avoid downtime. 

During this stage, check that you’ve properly configured your source environment. (And of course, you’ll need to verify that you have the permissions to do that work!)  

Your source tenant must also create at least one mail-enabled security group. These groups allow you to manage access permissions and email distribution and, in the context of your migration, limit or control which mailboxes can be moved. 

Also, check whether you need to fulfill any compliance or legal requirements before the migration. 

Step 4: Prepare your target environment 

Correctly setting up and configuring your new home will also be a critical step to ensure that you can seamlessly transition your mailboxes and that your end users have a productive experience. 

This will include tailoring your new tenant’s security settings policies, as well as provisioning user accounts with the appropriate licenses and permissions. You’ll also need to complete specific tasks, such as: 

  • Creating a migration application and secret (password), 
  • Establishing an Exchange Online migration endpoint and an organization relationship between your source and target tenant, and  
  • Setting up attributes for your target user objects. 

Step 5: Get your business users ready 

Last but certainly not least, you can nip risks in the bud by preparing your users for the migration. Doing so will also help you manage their expectations and foster confidence in the process, helping them understand their role in protecting your data’s integrity. 

First off, make sure to inform your users about the upcoming migration. No one likes a sneaky surprise!  

Any communication should include as many details as possible about your projected timeline and the potential impacts they could encounter. Give them crystal-clear instructions on how to navigate any changes they might experience during the process. And provide any necessary training alongside setting up support channels to help them address any issues they may face. 

To help you start that conversation, we’ve created a sample email template that you can tweak to fit your own cross-tenant mailbox migration. 

Pre-migration E-mail 

When to send: 4 weeks before migration 

Purpose: Raise awareness about your Exchange Online mailbox migration with your users 

Subject: Upcoming changes to your email service 

Hi team, 

We’ll be transferring our Exchange Online subscription to another tenant on [migration date]. The goal for this migration is to…[include details about your specific context here.] 

What to expect 

Our team has been plugging away behind-the-scenes to ensure that you experience a seamless transition to your new environment. This includes preparing your mailbox data to be migrated so that you experience little to no disruption to your work during the process. On your end, there’s nothing you need to do right now.  

Over the next few weeks, we will regularly update you on our progress. We’ll be holding a virtual event on [date] where you can join us to learn more and get your questions answered on the spot. Look out for an invite! 

In the meantime, if you have questions or concerns about this move, don’t hesitate to contact [admin name/phone # or e-mail] or refer to our Migration FAQs here: [link] 

Thanks for your support and cooperation, 

Your IT team 

Migrating mailboxes with Microsoft native tools 

As with any technology-related situation, there’s no such thing as a cookie-cutter cross-tenant mailbox migration. When selecting the tool(s) you’ll use to execute this project, your decision will depend on various factors, such as the size and complexity of the project and what tools are right for your mailbox migration needs. 

Microsoft’s built-in tools can do the job if you’re comfortable with coding. Yet they’re limited in supporting larger-scale migrations or more intricate scenarios and can be complex. They require a lot of elbow grease for manual intervention and oversight, which means there’s a chance of running into errors along the way. You’ll also need to stay updated on the new features Microsoft may be testing in public previews, which could change how mailbox migration works altogether. All this eats up a lot of your time that could’ve been spent on more strategic tasks. 

Microsoft offers a tool currently in public preview, where you can pay extra to migrate mailboxes from tenant to tenant. However, a fully automated migration tool like ShareGate can have the additional advantage of making your mailbox migration simple and fast, offering more nuanced customization options, as well as other advanced features such as automation, scheduling, and detailed reporting.  

Steps to complete a successful mailbox migration with ShareGate 

As part of our migration tool, ShareGate has a “copy mailboxes” feature to help you easily set up a Microsoft 365 cross-tenant migration. All you need to do is set up the right permissions and user licenses, and then you’re good to go.

To copy mailboxes using ShareGate:

  • Select Copy from the sidebar 
  • Select the Copy mailboxes tab. 
  • Click Copy mailboxes
  • Select your source tenant. 
  • Select the mailboxes you want to migrate. 
  • Select your destination tenant. 
  • Customize your copy options around your emails, calendars, contacts, and mailbox rules. 
  • Map your mailboxes 
  • Map your recipients 
  • Review a summary of your migration plan. 
  • And then, start your migration! 
Mailbox Copy Phase 2 Flow

Best of all, you can download a detailed copy report to understand your completed migration, including details about any warnings or errors that might have occurred so you can solve these issues for your users.

Dive deeper into the benefits in this guide to Microsoft 365 tenant-to-tenant mailbox migration with ShareGate.

Post-migration steps 

Once you’ve moved your mailboxes, your work isn’t finished! During the post-migration phase, you need to make sure that your new environment is secure, smoothly functioning, and compliant with any data regulations. To accomplish that, you can:  

Establish a governance plan 

When you’re prepared from the start, minimizing a high-risk situation like a migration becomes easy-peasy.   

Before your migration, focus on outlining policies and procedures regarding data access, sharing, and security protocols. By doing so, you’ll be more likely to avoid losing critical data or straining your IT resources. 

Your organization should also set up guardrails to keep the process on track and help you promptly identify and address potential problems. This may involve developing contingency plans for various potential migration setbacks and planning for conducting regular audits to ensure you’ve mapped data correctly. 

Monitor user adoption 

The last thing you want is for your employees to encounter barriers to their productivity post-migration. To steer clear of that outcome, you’ll need to proactively establish mechanisms for monitoring user adoption. Then, look at that data regularly to see if you can identify any trends.  

Your usage analytics will offer you a treasure trove of insights that you can use to pinpoint any difficulties users are struggling to navigate or whether there are any features or functionalities that they need to be trained on. 

Provide training and support 

To help your users experience a seamless transition, offer training and support post-migration. Make sure that they know they can rely on you to address any questions or concerns that may come up during the transition period. This is particularly important if your destination tenant looks different from your previous environment and requires new knowledge to navigate.  

Doing all of this will allow you to keep email communication flowing uninterrupted. While complex, you can successfully execute a cross-tenant mailbox migration with careful planning and the right tools and resources by your side. You can’t go wrong when you prioritize optimizing your user experience at every step of the way.  


How long does mailbox migration take?  

Each migration scenario is unique! So, of course, the length of time it will take you to execute your mailbox migration will vary. It will depend on your mailbox size, the number of items you want to shift, and the complexity of any rules, permissions, and shared folders. Your network speed and bandwidth will also affect the data transfer, along with your server’s current load and potential resource constraints. 

On top of that, the migration method you choose will affect the timeline, depending on whether you choose to do the entire process in one shot or over time. Of course, meticulous planning will be key for shorter migration timeline too! 

And finally, your tool choice will have a significant impact. When you use a third-party tool like ShareGate, you cut out a lot of time that you would otherwise spend on coding solutions.  

Are there any disruptions during migration? 

Proper planning can help you avoid any major disruptions to your operations during the migration process. 

It’s important to be aware that errors in the initial mapping stage could temporarily make emails inaccessible to users and cause delays in the flow of emails to and from their inboxes. In other cases, users may find that calendar events and contacts aren’t syncing instantly or that their server-side rules and filters need to be recreated.  

Using a hassle-free migration tool like ShareGate and implementing best practices can allow you to copy your mailbox with ease. 

How do mailbox permissions transfer post-migration?    

Certain permissions will come along during a cross-tenant mailbox migration, if you take specific actions to set up the right parameters. In other cases, not all permissions can be transferred between tenants. For example, the Mailbox Permissions that you store in a particular mailbox
will move alongside it when being transferred, as long as the principal (the owner of the mailbox) and the delegates (the particular users that you permitted to access the mailbox) are both moved to the target tenant. In this context, these would be permissions around who is allowed to open the mailbox, view its contents, or add or remove them. 

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