Migrating 100,000+ mailboxes to Microsoft 365
A large, multi-site organisation moved off ageing on-premises Exchange with near-zero downtime and no data loss.
A large, multi-site organisation needed to move off ageing on-premises Exchange without interrupting a workforce that lives in email. We delivered it in scheduled batches with near-zero downtime and no data loss.
// the challenge
Move a huge mailbox estate without stopping the business
The organisation ran legacy on-premises Exchange at a scale where a single misstep would hit tens of thousands of users at once. Mailboxes carried years of mail, shared mailboxes, public folders, calendars, delegations and rules that all had to survive the move intact.
A big-bang cutover was off the table. The migration had to happen alongside normal operations, with users continuing to send and receive throughout, and with a clear way to pause or roll back if anything looked wrong.
// what we did
Our approach
Discovery
Full audit of the Exchange estate, identity, mail flow and dependencies, with mailboxes grouped into low-risk migration batches.
Coexistence
Hybrid configuration so on-premises and Exchange Online ran side by side, with mail routing intact during the whole rollout.
Batched moves
Mailboxes migrated in scheduled, monitored batches. Each wave was validated before the next, with rollback ready at every step.
Cutover & cleanup
Per-user cutover was near-instant. Once complete, legacy Exchange was decommissioned and the tenant hardened.
// results
The outcome
- Over 100,000 mailboxes moved to Exchange Online with no data loss.
- Near-zero downtime per user, with no interruption to day-to-day email.
- Shared mailboxes, public folders, calendars and permissions preserved.
- Legacy Exchange retired, lowering cost and maintenance overhead.
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