Migrating 5,000 Mailboxes with Zero Downtime
A phased Microsoft 365 to Google Workspace migration for an organization of roughly 5,000 mailboxes, with full historical mail data preserved.
Challenge
Moving roughly 5,000 mailboxes from Microsoft 365 to Google Workspace in two weeks, without downtime, was only half the problem. The harder part was proving that no mail, calendar, or contact data was lost in the process. At this volume, a migration can look complete and still be silently missing data. That meant migrating in careful phases rather than a single cutover.
Approach
We migrated mailboxes in phases rather than all at once, using Google Workspace's Data Migration Service for the core mail transfer, with ShareGate supporting the migration workflow alongside it. DNS and MX records were cut over and verified with MxToolbox at each phase to confirm mail flow was routing correctly before moving the next group.
To answer the "did we lose anything" question, we cross-checked the migrated data against the source throughout the process rather than relying on the migration tools' summaries alone. That caught a small number of recurring calendar events that had initially gone missing; they were identified and resolved during validation.
Result
- ~5,000 mailboxes migrated over two weeks, in phases, with zero downtime
- Full historical mail data preserved
- Migrated data verified against the source, including the recovered calendar events