Coexistence During Office 365 Migration

When an organization can’t move every mailbox to Microsoft 365 in a single weekend, coexistence becomes part of the migration plan. Coexistence is the period during which some users are already on the new platform while others remain on the old one — yet email, calendars, and free/busy lookups still need to work seamlessly between them. Getting this wrong is one of the most common reasons Office 365 migrations run into delivery failures, missing meeting invites, or duplicate mailboxes.

EdbMails Office 365 Migration is built to minimize the need for long, complex coexistence periods in the first place — by migrating mailboxes, public folders, and Microsoft Teams data quickly and incrementally, so organizations spend less time stuck between two systems. This page explains what coexistence actually involves, the methods available, and how to plan for it if your migration must happen in phases.

What Is Coexistence in an Office 365 Migration?

Coexistence refers to the technical and operational state in which two messaging environments — for example, on-premises Exchange and Exchange Online, or two separate Microsoft 365 tenants — continue to communicate correctly while users are migrated in batches rather than all at once. During this window, migrated and not-yet-migrated users must still be able to:

• Send and receive email from each other without bounces or delays
• Appear correctly in the Global Address List (GAL)
• See accurate free/busy and calendar availability for scheduling meetings
• Access shared resources such as distribution lists and shared mailboxes

Coexistence is typically required for staged migrations, hybrid Exchange deployments, and tenant-to-tenant migrations, where the migration is expected to run over several weeks or months rather than completed in a single cutover.

Why Coexistence Is Needed

Not every organization needs a coexistence strategy. Smaller environments often perform a cutover migration, moving everyone at once over a short maintenance window. Coexistence becomes necessary when:

• The mailbox count is too large to migrate in one pass without disrupting business operations
• Migration is happening gradually, department by department or region by region
• Two organizations are merging and operating on separate tenants for a transition period
• Compliance, licensing, or data residency requirements demand a phased approach
• The business cannot tolerate downtime or broken communication during the transition

Core Components of Coexistence

1. Mail Flow Coexistence

This is the most critical piece. Mail addressed to a user must route correctly whether that user has already been migrated or not. Common approaches include:

• Internal relay domains – the source mail system is configured to relay messages intended for already-migrated users onward to Microsoft 365.
• Address rewriting/routing rules – mail flow rules or connectors detect which mailboxes have moved and forward messages accordingly, often using a shared SMTP domain across both environments.
• Hybrid mail flow via Hybrid Configuration Wizard – for on-premises Exchange to Exchange Online migrations, Microsoft’s Hybrid Configuration Wizard sets up the necessary connectors, accepted domains, and organization relationships automatically.

2. Directory and GAL Synchronization

Users on both sides need to see each other in the address book with correct contact details. This usually relies on directory synchronization tools (such as Microsoft Entra Connect, formerly Azure AD Connect) so that mail-enabled users, distribution groups, and contacts stay consistent across environments throughout the migration.

3. Free/Busy and Calendar Coexistence

Scheduling meetings across migrated and non-migrated users requires a working availability lookup. In hybrid Exchange environments, an organization relationship or federation trust enables free/busy sharing. In tenant-to-tenant or cross-platform scenarios (for example, Google Workspace to Microsoft 365), tools like Calendar Interop or third-party connectors are used to keep availability information visible on both sides.

4. Distribution Groups and Shared Resources

Distribution lists, shared mailboxes, and meeting room resources must remain usable by both migrated and not-yet-migrated users. Many organizations manage this temporarily through dynamic distribution groups or scripted membership updates during the migration window.

Coexistence Approaches: A Comparative Overview

How EdbMails Reduces the Need for Long Coexistence Periods

EdbMails Office 365 Migration Tool is designed around incremental, delta migration rather than long parallel-running coexistence:

• Batch-wise migration lets you move mailboxes in manageable groups, shrinking the coexistence window per batch instead of running an extended, organization-wide coexistence phase.
• Delta/incremental sync re-scans already-migrated mailboxes and copies only new or changed items, so users can keep working on the source system until the final cutover with minimal data lag.
• Automatic mailbox mapping and pre-migration discovery reduce the manual GAL and mailbox-matching work typically needed to keep both environments in sync during coexistence.
• Direct migration of mail, calendars, contacts, and tasks to Microsoft 365 — including for tenant-to-tenant, Exchange-to-Office 365, and IMAP-based source environments — means calendar and free/busy data exists natively on the target as soon as a batch completes, reducing reliance on a separate free/busy bridge for migrated users.
• Pre-migration and post-migration reports give visibility into what has and hasn’t moved, helping you decide when it’s safe to redirect mail flow or retire coexistence routing for a batch.

This approach won’t replace a hybrid hub-and-spoke for organizations that genuinely need on-premises Exchange to persist for years — but for most staged, tenant-to-tenant, or platform-to-Office 365 moves, it shortens the time mail administrators have to spend maintaining dual-running systems.

Best Practices for Managing Coexistence

1. Keep the coexistence window as short as possible. The longer two systems run in parallel, the more opportunities for misrouted mail and inconsistent calendars.
2. Migrate in logical batches — by department, location, or team — so that closely collaborating users move together and calendar/free/busy issues are minimized within a working group.
3. Lower DNS TTLs in advance of any MX or domain cutover to reduce propagation delays.
4. Test free/busy and mail flow with a pilot group before migrating the wider organization.
5. Communicate migration batch schedules to end users so they know what to expect during the transition.
6. Clean up coexistence artifacts — relay domains, temporary connectors, transport rules — once migration is complete, to avoid leaving fragile, forgotten configurations in production.
7. Validate distribution groups and shared mailboxes after each batch to confirm membership and permissions carried over correctly.

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Additional resources:

• EdbMails Office 365 Migration Software
• Office 365 Tenant-to-Tenant Migration Guide
• Public Folder Migration to Office 365
• How to validate Office 365 mailbox migration success
• EdbMails Vs Competitors comparison