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Reading note: This article focuses on Exchange Online to Exchange Online migration (tenant-to-tenant scenario) as the detailed practical case. ShareGate covers many other workloads (SharePoint, OneDrive, Teams, Planner, Entra ID identities), but covering each in depth would make the article endless. The goal here is to give a clear view of the tool, then dive into a concrete step-by-step walkthrough of mailbox migration.
1 – Introduction
Migrating a Microsoft 365 environment whether from on-premises Exchange, from another provider, or from one tenant to another during a merger or acquisition remains one of the most sensitive operations in the life of an infrastructure. Microsoft’s native tools (Exchange Online PowerShell cmdlets, Migration Batches, Cross-Tenant Mailbox Migration) get the job done, but at the cost of heavy manual configuration, limited reporting, and a non-negligible margin of error as soon as volume grows.
ShareGate Migrate positions itself in this space : a third-party tool that industrializes M365 migration while adding a layer of visibility (graphical mapping, audit and security reports) that native tools don’t offer out of the box. After several days testing it on a real-world scenario, here is my feedback, strengths and limitations included.
2 – Overview : What ShareGate Migrate Covers
ShareGate Migrate isn’t limited to mailbox migration. The application is built around two broad usage modes : Migration and Governance / Reporting. Before getting into the hands-on, here’s a panorama of the capabilities, because this is a strength that would be a shame to overlook.
2.1 – Available migration types
The interface groups all scenarios under the Copy tab, with a clear categorization logic :
| Category | Workloads covered |
|---|---|
| Copy existing structure and content | SharePoint sites, lists/libraries, content types, columns, SharePoint groups, permission levels, workflows, managed metadata, sensitivity labels |
| Category | Workloads covered |
|---|---|
| Copy mailboxes | Exchange Online mailboxes (emails, attachments, folders, calendars, contacts) — and import from Gmail to Exchange Online |
| Category | Workloads covered |
|---|---|
| Copy identities (Preview) | Users, groups, and memberships in Entra ID |
| Category | Workloads covered |
|---|---|
| Import external content | Import from a file share or from Google Drive (My Drive + shared drives) to SharePoint / M365 / OneDrive |
| Category | Workloads covered |
|---|---|
| Restructure environment | Promote a subsite into a site (subsite to site collection) |
What stands out from this layout is that ShareGate addresses both “Migrate to Microsoft 365” migrations (from on-prem, file shares, Google Workspace) and “Tenant-to-tenant” migrations (merger/acquisition, divestiture, consolidation). The same console handles both logics, which avoids juggling multiple tools.
2.2 – Beyond migration : audit and governance
This is an aspect that particularly intrigued me. ShareGate isn’t just a copy engine : it’s also a reporting and security platform. Two distinct sections demonstrate this.
All reports offers a library of ready-to-use reports, complemented by the ability to create custom reports :
- Permissions matrix report (who has access to what)
- Audit report (log of operations by user/target)
- Site report / Site collection report
- Checked out documents report
- Sites with custom permissions report
- Unused site report (sites inactive for 6 months)
- External user report / External sharing report
- Orphaned user report
- Lists with workflows report / Workflow report
- Teams summary report / Planner plans summary report
Security highlights reports oriented toward security posture, led by the Permissions matrix report, accompanied by Audit, Orphaned user, External sharing, Sites with custom permissions, and External user reports.
ℹ️ Important scope note : the reporting and security capabilities are heavily oriented toward SharePoint and OneDrive. The vast majority of built-in reports (permissions matrix, site and site collection reports, checked-out documents, sites with custom permissions, external sharing, orphaned/external users) operate at the SharePoint/OneDrive layer. Teams and Planner get summary-level coverage (Teams summary report, Planner plans summary report), but Exchange Online / mailbox-level governance is not the focus here, don’t expect mailbox-centric audit or security reporting comparable to what you’d get for SharePoint sites. If your governance priority is messaging and mailbox compliance, you’ll still need Microsoft Purview, the Exchange admin center, or a dedicated tool alongside ShareGate.
Key takeaway : even outside of any migration project, ShareGate Migrate can serve as a tool for continuous governance, mapping permissions, detecting risky external sharing, or identifying orphaned accounts. That’s a strong argument for amortizing the license over time, not just over the window of a migration project.
3 – Hands-On : Exchange Online to Exchange Online Migration (Tenant-to-Tenant)
Now to the practical case. The chosen scenario is a cross-tenant migration of Exchange Online mailboxes, typical of a merger/acquisition or tenant consolidation.
The goal : transfer emails, folders, calendars, and contacts from the source tenant to the target tenant with minimal downtime.
3.1 – Prerequisites and preparation
Before launching anything, a rigorous preparation phase determines 80% of a migration’s success.
Licensing and access
- A valid ShareGate Migrate license (Insane / High Speed Migration mode recommended for large volumes).
- Administrative accounts with sufficient rights on both tenants (source and target). An account with the Global Administrator role or, at minimum, Exchange Administrator combined with the required application rights.
- Target mailboxes must already exist in the destination tenant (user provisioning and Exchange Online license assignment done upfront).
Identities and domains
- A defined source-to-target user mapping strategy (see dedicated section below).
- A decision on the fate of SMTP addresses: keep the domain, add it as a secondary address, or fully switch over.
- If the domain is to follow, plan the domain transfer between tenants (a step outside ShareGate, but to be planned in the overall runbook).
Assessment (not to be neglected)
- Run an inventory / assessment of source mailboxes: volume, item count, presence of shared mailboxes, rooms, resources.
- Identify large mailboxes (>50 GB) that will take longer and potentially require pre-seeding.
Step 1 – Connecting the source tenant and select the source mailbox
Under the Copy → Copy mailboxes tab, select the Copy mailboxes (Exchange Online) tile.
- ShareGate asks you to authenticate to the source tenant.
- Modern authentication (OAuth / modern auth) is used (depending on your Conditional Access configuration, plan an exclusion or an approved app for the service account used by ShareGate, otherwise Conditional Access policies risk blocking the connection).
- You need to read and agree new entra id app authorization by clicking on “Accept“.
- Select Source mailbox (one or many, but ShareGate recommends up to 16 per JOB per machine. Check the best practice section for all details)
Step 2 – Connecting the target tenant
keep in mind , destination mailbox should be existing or created before starting migration process.
Once source tenants is connected, do the same to connect destination tenant.
- Once destination tenant is connected, select what you want to migrate (Emails, Calendars, Contacts…etc).
- Clic “Continue to mailbox mapping“.
Step 3 – Mapping source and destination mailboxes (the strong point)
This is where ShareGate truly stands apart from native tools. The graphical mapping lets you automaticaly associate each source mailbox with its target visually basing on intelligence algorithme, without writing a single line of PowerShell.
Concretely, you can:
- Map users source to target (useful when UPNs change between the two tenants, which is almost systematic in cross-tenant scenarios).
- Map SMTP addresses to preserve or rewrite addresses.
This “user-friendly” approach drastically reduces the risk of mapping errors , one of the most frequent causes of failure in manually driven cross-tenant migrations.
- A second window for mapping recipients will also appear after the previous window (it works the same way and follows the same principle : source mailbox and destination mailbox)
💡 Best practice : On cross-tenant migrations, export your mapping table and have it validated by the business before cutover. A mapping error on an executive or a critical functional mailbox is costly.
Step 4 – Pre-migration validation
Once the source and destination mailboxes are mapped, ShareGate shows a job summary before you start , recapping the source-to-destination pairing, flagging issues that may block a complete copy (e.g. Single Item Recovery or unmapped items), and letting you fix any misconfiguration before clicking Start copy.
Click “Start Copy“
Step 5 – Processing with mailbox copying
Once you click Start copy, ShareGate begins the migration job and opens the Copy Summary Report, which gives you a live, at-a-glance view of progress as the mailboxes are copied.
The report header confirms the source-to-destination pairing (here, GlobaliTnow→ Cloudnsecops, both Exchange Online) and shows the overall job status (Completed once finished). Three key counters summarize the run in real time:
- Migrated size — the total volume of data copied.
- Migrated mailboxes — how many mailboxes have been processed.
- Migrated items — the cumulative number of items (emails, calendar entries, contacts) transferred.
- Once migration finished :
Step 6 – Post-migration validation
The migration doesn’t end with the transfer. ShareGate generates a detailed, exportable migration report (typically downloaded as an Excel/CSV file) that must always be reviewed , it’s your single source of truth for what actually happened during the copy, and the foundation of any serious validation and handover.
Rather than a simple pass/fail summary, the report logs every single object processed during the job, one row per item.
⚠️ Don’t forget : shared mailbox permissions, delegations, and transport rules don’t always carry over automatically depending on the scenario. Verify them explicitly in your post-migration validation plan.
4 – Best Practices
Plan around Microsoft throttling from day one :
- Always run a pilot migration on a sample of 5 to 10 representative mailboxes (a standard user, a high-volume user, a shared mailbox, a mailbox with delegations) before scaling up. This reveals mapping and throttling issues without impacting all users.
- Keep in mind that mailbox migration speed is largely outside your control Microsoft enforces service-protection throttling to preserve platform stability for all tenants. In practice you’ll hit limits on concurrent mailbox moves and per-mailbox ingestion rate well before your bandwidth becomes the bottleneck. A common real-world figure is roughly 1.5 GB/hour per mailbox under throttling, and ShareGate’s own planning estimate is around 10 GB/hour per machine, with wide variation depending on item count, content complexity, and how aggressively Microsoft throttles. ShareGate’s Insane Mode (which leverages Microsoft’s Migration / Import API) is the recommended setting because it resists throttling considerably compared to standard APIs.
- Because a single machine quickly plateaus against these limits, the proven way to gain time is to run several ShareGate instances in parallel across multiple machines or VMs, each with a distinct workload (different sets of mailboxes) and ideally a distinct admin account, to spread the load and reduce throttling. This multi-machine capability is gated by license tier: ShareGate Migrate Pro supports 5 machine activations, and Enterprise supports 25. There is no built-in coordination between machines, each instance runs independently with its own local history and database, so you must partition your mailbox batches manually to avoid overlap and duplication (you can use shared Excel file or SharePoint List).
- Additional levers to minimize throttling and downtime : enable Insane Mode, consent to the Azure application in ShareGate to reduce request overhead, and schedule migrations outside business hours (overnight or weekends) when tenant load is lowest.
5 – Strengths and Limitations : My Verdict
5.1 – What I liked
- One app for the entire M365 migration : ShareGate covers every facet of M365 migration in a single application, Exchange, SharePoint, OneDrive, Teams, Planner, identities, file shares, Google Workspace, instead of juggling a separate tool for each workload as you often must elsewhere.
- Very user-friendly interface : the UI is exceptionally intuitive , self-documenting Copy tiles that list exactly what’s copied and clear navigation keep the learning curve low, so even an L2 technician is productive almost immediately.
- Convenient mailbox mapping : this is the major differentiator versus native tools. Mapping users and properties visually avoids errors and speeds up preparation, especially in cross-tenant scenarios.
- Incremental approach : pre-migration + delta = minimal downtime, perfectly suited to production environments.
- Dual migration + governance role : the audit and security reports make the tool much more than a simple copy engine, though this governance value is concentrated on the SharePoint/OneDrive layer (see limitations). For SharePoint-heavy estates, it doesn’t stop being useful at the end of the project.
- Clear migration reporting : tracking copied/skipped/error items facilitates validation and targeted re-runs.
5.2 – Limitations
- Cost : ShareGate targets mid-market and enterprise companies (such as companies with 200 or more users) or companies that specialize in acquiring other businesses and that’s where having a ShareGate license becomes very valuable.
- Dependence on tenant prerequisites : ShareGate doesn’t relieve you of target provisioning, domain management, or Conditional Access configuration. Planning remains your responsibility.
- Permissions/delegations not systematically transferred : depending on the scenario, some properties require manual post-migration verification.
- Preview features : Entra ID identity copy is still in preview, to be handled with caution in production.
5.3 – Who is it for ?
ShareGate Migrate is aimed above all at IT teams and MSPs handling medium-to-large M365 migrations, and particularly at tenant-to-tenant scenarios (merger/acquisition, consolidation) where the complexity of mapping and the need for reporting fully justify the investment. For an SMB performing a simple, one-off migration, the cost/benefit analysis is worth posing against native tools.
6 – Conclusion
ShareGate Migration delivers on its promises for the use case I tested: cross-tenant Exchange Online to Exchange Online migration is noticeably smoother, more visual, and less risky than a native PowerShell approach, mainly thanks to graphical mapping and the incremental approach. The governance layer (audit, security, permissions) is a lasting added value that goes well beyond the scope of a one-off project.
It’s not a magic solution, it doesn’t relieve you of rigorous planning and a serious validation phase, but it’s a serious accelerator for anyone who needs to industrialize M365 migrations without sacrificing control or visibility.
For more details about ShareGate Migrate visite this link : https://sharegate.com/solutions/mailbox-migration
Thanks.
