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

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.