Business Problems

In the wake of a corporate merger, the organization initiated a Microsoft 365 cross-tenant migration to consolidate IT resources and streamline user collaboration between the parent company and its acquired subsidiary.

Project Plan

Migration Tools and Licensing
To facilitate this complex migration, the team used the Microsoft Native Cross-Tenant Migration Tool with the following resources:

New Microsoft 365 licenses procured for the new (parent) tenant.
Cross-Tenant Migration licenses allocated for the process.
App Registration created to enable secure access via APIs.
Migration Endpoints were configured to enable data synchronization and migration execution.
User mapping was done to ensure accurate migration paths between the source and target tenants.

Migration Scope

Mailboxes: Full mailbox migration including inbox, folders, and subfolders.
Calendars: All events including recurring meetings and attachments.
Contacts: Complete address books per user.
Domain Migration: Domain settings transitioned to the parent tenant.

Domain Configuration & DNS Updates
To retain the existing child company domain in the parent tenant, the following steps were taken:

SPF (Sender Policy Framework): Updated to reflect new mail servers.
MX Records: Redirected to the parent tenant’s mail flow.‍
DMARC (Domain-based Message Authentication, Reporting &Conformance): Configured for email authentication and policy enforcement.

Technical Steps

Domain Preparation:
- Verified and removed the domain from the child tenant.
- Added and verified it in the parent tenant.
- DNS records updated accordingly (MX, SPF, DMARC).
App Registration & Permissions:
- Registered an Azure AD application with required Graph API permissions.
- Admin consent granted to enable tenant-wide access for migration tools.
Migration Endpoint Setup:
- Defined migration endpoints for both source and target tenants.
- Configured throttling policies and ensured adequate bandwidth.
User Mapping & Validation:
- Mapped source users to their corresponding identities in the new tenant.
- Ran pre-migration validations and pilot tests.
Migration Execution:
- Executed migrations in controlled batches.
- Monitored and validated data post-migration.
- Provided rollback procedures for any anomalies.

Benefits

All selected users were successfully migrated with minimal downtime.
Emails, calendars, contacts, and rules were intact and functional in the new tenant.
Users retained the same email addresses and domain names.
The transition was transparent to most end-users, requiring minimal training or support.
The project served as a blueprint for future M&A integrations within the organization.

Key Takeaways

Using native Microsoft tools offers a cost-effective and compliant approach for M365 migrations.
Proper DNS planning and domain transfer sequencing is critical in domain retention scenarios.
Pre-migration testing and user mapping play a vital role in avoiding post-migration issues.
Ensuring app registration with correct permissions and endpoint configuration is essential for a smooth API-based migration.

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