This guide is part of our M365 Migration Series.
Explore every step of the tenant-to-tenant migration journey below:
Mergers and acquisitions often force the consolidation of IT environments. For many organizations, especially during IT Services Mergers and Acquisitions
projects, one of the critical steps is migrating Exchange Online to the acquiring tenant.
This blog will outline the general steps needed to migrate mailboxes with the goal of seamless coexistence during migration and minimal downtime. This blog will also focus on using Microsoft-native tools (read: free/included).
Step 1: Planning & Assessment
Before touching any configuration, invest a significant amount of time in planning.
- Inventory mailboxes: inventory your user, shared, and resource mailboxes. Determine what is going to migrate and what may not (i.e. disabled accounts, mailboxes in retention, etc.). Always pay special attention to VIPs and service accounts.
- Domain strategy: determine whether the acquired company’s domain will to the acquiring tenant. This could impact the timing of the DNS cutover.
- Identities: ensure Azure AD identities are mapped. You should consider using Entra ID cross-tenant synchronization to simplify identity management.
- Licensing check: ensure that the acquiring tenant has enough Exchange Online licenses of the correct type (P1 vs. P2).
- Timeline: plan for a phased migration over weeks, not months. The longer the migration persists, the more risk there is for duplicate data, client confusion, and overall dissatisfaction with the move.
Step 2: Coexistence Setup
Coexistence ensures your clients can continue sending, receiving, and scheduling meetings during the migration.
- Cross-Tenant Mailbox Migration: Microsoft’s Mailbox Replication Service (MRS) now supports tenant-to-tenant moves. This is the backbone of the migration process.
- Mailflow coexistence: configure either dual delivery or mail routing via accepted domains and use mail user objects in the target tenant to represent source mailboxes until migration is complete.
- Free/Busy sharing: enable cross-tenant calendar availability so clients can still schedule meetings between the two tenants.
- Authentication & permissions: establish a trust between the tenants using cross-tenant access settings in Entra ID.
Step 3: Migration Execution
Prepare the Acquired Tenant
- Enable the mailbox migration endpoints.
- Create a service account in both tenants. Assign the service account the Migration Administrator role in Exchange Online (this is an internal Exchange role, not an Entra ID role).
- Validate network connectivity and throttling policies.
Prepare the Acquiring Tenant
- Create migration batches using either Exchange Admin Center (EAC) or PowerShell.
- Validate accepted domains and DNS records.
- Provision Mail User objects for coexistence using either Exchange Admin Center (EAC) or PowerShell.
Pilot Migration
- The first migration wave should only be two or three mailboxes. This batch is just to verify coexistence and free/busy sharing is working correctly.
- The second migration wave should be 10-20 mailboxes. In this wave, consider including:
- At least one executive-to-admin assistant pair. This helps to verify that mailbox delegation worked as expected.
- At least one shared mailbox and resource mailbox.
- Run one more smaller wave of maybe 100 or so mailboxes to get additional feedback. This should be power users and clients who will give you honest, helpful feedback.
- Use these waves to validate mailflow, calendar sharing, and Outlook profile behavior.
- Gather feedback from your pilot clients and adjust your plans accordingly.
Full Migration
- Split the remainder of your mailboxes into waves and run them.
- The best way to monitor migrations is via PowerShell, as the UI lacks some helpful detail. Here is a simple PowerShell script that will give you some insight into the migration:
Connect-ExchangeOnline
Get-MigrationBatch
Get-MigrationUser
- Try to address failures promptly so that your clients do not lose confidence in the migration process.
Post Migration
- Move the primary SMTP domain to the acquiring tenant.
- Update DNS records: MX, Autodiscover, SPF, DKIM, DMARC, etc.
Step 4: Post-Migration Validation
After your migration is complete, validate thoroughly:
- Client testing: confirm that Outlook, OWA, and mobile clients connected for clients without issues.
- Mailflow checks: validate inbound/outbound routing.
- Calendar & delegate permissions: make sure that permissions are preserved or re-applied.
- Decommission source tenant mailboxes: remove migration endpoints and any remaining mail users from the acquiring tenant
Recommended Strategy
- Phased migration with coexistence: avoid “big bang” cutovers unless your migration is very small. Migrate in manageable batches to reduce risk, manage client expectations, and manage post-migration support.
- Microsoft-native tools first: Cross-tenant mailbox migration (MRS-based) is the recommended approach to save on costs.
- Third-party tools (optional):
- Quest Migration Manager or BitTitan MigrationWiz can help with advanced reporting, automation, or hybrid coexistence features if necessary, but come at an added cost.
- These are useful for very large or complex environments but may not be necessary for mailbox-only migrations or small or mid-size migrations.
Best Practices
- Communicate early and often with your clients.
- Run pilots before your first large-scale migration.
- Keep coexistence running until all mailboxes are validated.
- Monitor logs and migration reports often.
- Plan DNS cutover during low-traffic windows and be sure to consider the TTL of the DNS entries.
- Document every step for audit and rollback.
Conclusion
With proper planning, mailbox migration between tenants can be completed in weeks, not months. By using Microsoft-native tools, you can significantly cut down on costs. Coexistence ensures minimal disruption to mailflow or calendaring during the transition. A phased approach balances speed with stability, making it the best fit for M&A scenarios. especially during larger IT Services Mergers and Acquisitions where multiple environments need to be consolidated smoothly.
By following these steps, you can deliver a smooth migration experience for your clients.
Modernize Your Business With a Seamless Microsoft 365 Migration
Simplify your move to the cloud with expert planning, native tools, and proven M&A migration strategies that ensure zero disruption.
Talk to Our Migration Experts
