We are preparing for a Microsoft 365 tenant consolidation, and one area that needs careful attention is mapping source users to their corresponding destination accounts. Our organization has employees across multiple departments, and some users will receive new email addresses after the migration. We also have shared mailboxes and service accounts that need to be handled separately. I want to understand how administrators can organize this mapping before starting an Office 365 tenant to tenant migration.

One issue is that source and destination accounts do not always have identical usernames or primary SMTP addresses. For example, an employee might use an older company domain in the source tenant but receive a new domain-based email address in the destination tenant. If the mapping is incorrect, mailbox data could be transferred to the wrong user or remain inaccessible after the migration.

I am also interested in how administrators should handle shared mailboxes, mailbox aliases, distribution groups, and user permissions. Should destination accounts be created and licensed before transferring mailbox content? What happens when multiple source accounts need to be consolidated into a single destination environment?

Another concern is validation. I would like to know which checks should be performed after migration to confirm that each user's emails, contacts, calendars, attachments, and folder structures have reached the correct destination mailbox. Maintaining a mapping spreadsheet and testing a small group of accounts before the full migration seem like useful preparation steps.

During my research, I found the SysInfo Microsoft 365 Tenant to Tenant Migration Tool, which offers source-to-destination mapping and migration options for different Microsoft 365 workloads.

For Microsoft 365 administrators, what is your preferred approach to user mapping during a cross-tenant migration? Do you maintain a CSV mapping file, configure accounts manually, or use an automated process? I would appreciate recommendations for avoiding identity mismatches and ensuring that every employee receives the correct mailbox data.
https://www.sysinfotools.com/o365-tenant-to-tenant-migration.php