Moving users to a new Microsoft 365 tenant is more than a mailbox project. IT teams also need to account for how employees find colleagues, access shared business contacts, and identify incoming calls on their smartphones. A successful mailbox move does not automatically mean every directory entry, contact workflow, and mobile address book is ready in the destination environment.
Quick answer: Microsoft supports moving user-visible mailbox content, including personal contacts, through its cross-tenant Exchange Online mailbox migration process. But the Global Address List (GAL) is built from directory recipient objects, not simply copied as a personal contact folder. Before cutover, confirm which directory objects must exist in the destination tenant. After cutover, test GAL search, shared contacts, and smartphone contact sync separately.
This guide explains what changes during tenant migration and gives Microsoft 365 administrators a practical contact-continuity plan for before, during, and after the move.
What is Office 365 tenant-to-tenant migration?
Office 365 tenant-to-tenant migration, also called Microsoft 365 cross-tenant migration, is the process of moving users and selected services or information from one Microsoft 365 organization to another. It commonly forms part of mergers, acquisitions, divestitures, reorganizations, or tenant consolidation.
In an Exchange Online mailbox migration, administrators prepare source and target organizations, establish the required permissions and relationships, provision the appropriate target objects, and move eligible mailboxes in planned batches. Microsoft documents this workflow and its licensing requirements. Other workloads, such as SharePoint, Teams, and identity configurations, have their own migration considerations.
The mistake to avoid is treating mailbox migration, directory preparation, GAL visibility, and phone contact synchronization as one operation. Each has different requirements and a different way to verify success.
What happens to contacts and the GAL during a tenant migration?
The answer depends on the kind of contact data involved. A contact stored in an employee’s mailbox is not the same thing as an organization-wide GAL entry, a shared contact, or a contact already saved on a phone.
| Contact or directory data | What administrators need to know |
| Personal mailbox contacts | Contacts stored as user-visible content inside a mailbox are included in supported cross-tenant Exchange Online mailbox moves. Local PST contacts or other client-only address books are separate. |
| Global Address List entries | The GAL itself is not copied as a mailbox contact folder. The destination GAL is generated from mail-enabled recipient objects and Exchange address-list visibility in the target tenant. |
| Distribution lists and mail contacts | These are not part of mailbox data migration. Microsoft advises creating the required objects in the target tenant separately. |
| Public Folder contacts | Public folders are not included in Microsoft FastTrack cross-tenant mailbox migration and require a separate migration or replacement plan. |
| Shared or departmental contacts | First identify where they are stored. If they live in another mailbox, Public Folder or separate contact source, validate that source’s migration path and permissions separately. |
| Smartphone contacts and caller ID | These are not validated by a successful mailbox migration. Test native phone contacts and caller ID separately after the destination contact-sync configuration is ready. |
For example, a migrated employee may see their own personal contacts in Outlook but still be unable to find every coworker in the destination GAL. Another employee might find a colleague through Outlook search, yet still see an unknown phone number on an incoming call because that colleague has not been synchronized into the phone’s contacts.
Before migration: prepare the directory and contact data
Contact continuity begins before the first migration batch. Document the current state so the team knows what must be transferred, recreated, synchronized, or tested.
- Inventory the source GAL. Identify mailboxes, mail users, mail contacts, distribution groups, shared mailboxes, and any custom address lists that employees depend on. Record critical phone numbers and other attributes that support daily communication.
- Separate personal and organizational contacts. Personal mailbox contacts may move with the mailbox through a supported Microsoft migration process. Other address book resources may need dedicated preparation.
- Map the destination directory. Confirm how employees and external contacts will appear as mail-enabled recipients in Exchange Online. Check expected email addresses, display names, phone attributes, and GAL visibility.
- Identify contact consumers. Include field teams, sales staff, help desks, executives, and any group that depends on smartphone caller ID or offline access to saved business contacts.
- Document current sync rules. If an organization uses automated GAL or shared-contact synchronization, capture the source, target users, contact folders, filters, and sync ownership. Plan how those settings will be changed when users move.
- Run a pilot. Migrate a representative user group, including people with personal contacts, shared contact access, and mobile devices. Confirm both data movement and everyday contact usability before expanding the rollout.
Microsoft requires specific target mail user preparation and cross-tenant migration licensing for its native mailbox moves. Check Microsoft’s current prerequisites for your migration approach instead of assuming permissions, provisioning, and licensing are already in place.
During migration: keep company contacts accessible
A staged migration can leave colleagues split between source and target tenants for days or weeks. Mail routing and mailbox access are important, but so is the ability to find the right person in the organization.
For every migration batch, use a documented communication and verification process:
- Confirm which people have moved and which still operate in the source tenant.
- Validate expected recipient objects and the address book experience in the destination.
- Test searches for employees, shared resources, and important external contacts from accounts on both sides where coexistence is required.
- Check that permissions and synchronization settings used for shared contact workflows still reference the correct accounts and resources.
- Tell employees what contact behavior may temporarily change, and provide an escalation route for missing contact details.
Be precise about coexistence. Microsoft Entra cross-tenant synchronization and Exchange Online GAL visibility are not identical features. A synchronized identity does not necessarily appear in Exchange address lists unless it meets the applicable Exchange recipient requirements. Test the complete workflow rather than relying only on an Entra directory count.
Avoid promising uninterrupted native smartphone contact syncing during an ongoing cutover unless you have tested and documented a supported configuration for that specific environment.
After migration: Validate the GAL and smartphone contact synchronization
The post-migration check should go beyond confirming that users can send email. Test whether employees can find and use up-to-date company contact information across Outlook and their everyday devices.
- Confirm mailbox contacts arrived. Open a pilot user’s Contacts folder and verify representative names, telephone numbers, and other fields after migration.
- Check the target GAL. Search for new and existing employees, contact-only recipients, departmental addresses, and any required shared resources.
- Validate shared contact access. Confirm that intended user groups can open and use the shared contact folders or other agreed company contact sources.
- Test native mobile contacts. On both iPhone and Android, check whether expected colleagues appear in the phone contact list and whether caller ID works for known internal numbers.
- Test a contact update. Change a controlled test record, allow the configured synchronization process to run, and confirm the updated data reaches the intended users without unwanted duplicates.
- Review stale entries and duplicates. Look for old tenant addresses, duplicate coworkers, inconsistent phone formats, and contacts that are no longer authorized for distribution.
- Assign ongoing ownership. Establish who will manage source records, contact synchronization configuration, security approvals, and support tickets after the migration team leaves.
A useful acceptance test is a short list of tasks a real employee performs: find a coworker, call that person from a phone, recognize an incoming call, and access a shared business contact. A green mailbox migration dashboard does not replace those tests.
Why contact continuity matters for mobile workforces
Contact gaps become especially visible in organizations with field technicians, sales teams, healthcare staff, service crews or other employees who work primarily from mobile devices. A mailbox can migrate successfully while employees still cannot identify an incoming internal call, find a colleague in their native phone contacts or access an important shared business contact.
For these organizations, contact continuity should be treated as a migration acceptance requirement rather than a post-migration convenience.
Where CiraSync fits into the migration plan
Neither CiraSync Cloud nor CiraSync Hub replaces Microsoft’s tenant-to-tenant mailbox migration process. They address contact synchronization requirements that can remain during or after the migration.
CiraSync Hub (CiraHub) is the relevant solution when the requirement is to synchronize contact data or Global Address Lists between multiple Microsoft 365 tenants. This can be useful during coexistence, mergers, acquisitions, or when multiple tenants will continue operating after the migration.
CiraSync Cloud addresses a different requirement: distributing approved GAL and shared contact data centrally to users. CiraSync synchronizes the selected contact information into users’ Microsoft 365 mailbox contact folders; those contacts can then become available on supported smartphones when mailbox contact synchronization is configured.
Some organizations may need both capabilities: CiraSync Hub to keep contact sources aligned across tenants, and CiraSync Cloud to distribute the resulting company contacts to employees’ Outlook and supported mobile contact experiences.
Before cutover, confirm which requirement applies and test the exact configuration with pilot users. Do not assume that an existing synchronization configuration will continue unchanged after tenant identities, permissions or source objects move.
Office 365 tenant-to-tenant migration contact checklist
| Stage | Validation checks |
| Before cutover | Create a representative test set containing employees, mail contacts, groups, shared contacts and important phone numbers. Record where each should appear: source GAL, target GAL, shared contacts and mobile contacts. Confirm the destination objects and run a pilot. |
| During cutover | For every batch, confirm migrated users can find expected coworkers, verify coexistence with users still in the source tenant, test important mail contacts/groups, confirm shared-contact access and test mobile contacts where required. |
| After cutover | Repeat the same test set. Verify mailbox contacts, GAL search, shared contacts, native phone contacts and caller ID. Change one controlled contact record and confirm the update reaches the intended users without creating duplicates. |
| Ongoing | Remove stale source-tenant entries, monitor synchronization failures and duplicates, confirm offboarding behavior and assign ownership for contact data and synchronization rules. |
Common problems after Microsoft 365 tenant migration
Problem: Personal contacts arrived, but users cannot find coworkers in the new GAL.
Check that the target Exchange Online directory has the expected mail-enabled recipient objects and that address list visibility is configured correctly. Mailbox contacts and the GAL are different data sets.
Problem: Users appear in Microsoft Entra but not in Exchange Online address lists.
Check Exchange recipient status and relevant synchronization settings. A cross-tenant synchronized identity alone is not proof of GAL visibility.
Problem: Outlook search works, but incoming mobile calls show unknown numbers.
Confirm whether the organization has deployed a supported method for synchronizing company contact records into the phone’s native contacts. GAL lookup and saved smartphone contacts are distinct experiences.
Problem: Shared team contacts disappeared or are duplicated.
Verify the source folder, permissions, target folder, mapping rules, and whether an old and new synchronization job are both distributing copies. Use a pilot group before removing old data.
Problem: Some contact records still contain old tenant details.
Review the authoritative source of the contact fields and the update process. Correct source records first, then validate propagation according to the configured synchronization method.
Keep company contacts usable after the migration
A successful tenant migration should leave employees with more than functioning mailboxes. They should be able to locate colleagues, recognize business calls, and use the company contact information they need to work. Build contact continuity into the migration plan early, verify it during cutover, and test it again after migration.
If your organization needs automated GAL and shared contact synchronization across users and supported mobile contact experiences, explore how CiraSync can fit into your Microsoft 365 contact management strategy.
Explore CiraSync | Request a demo
Frequently asked questions
Does Microsoft 365 tenant-to-tenant migration transfer contacts?
Microsoft’s supported cross-tenant Exchange Online mailbox migration moves user-visible mailbox contacts along with other eligible mailbox content. This does not mean all GAL, distribution group, and shared contact resources are automatically transferred.
Does the Global Address List migrate with Exchange Online mailboxes?
Not as a simple copy of each user’s personal Contacts folder. The destination GAL is derived from mail-enabled recipient objects and Exchange address list configuration. Plan and validate these separately.
Can Microsoft Entra cross-tenant synchronization replace GAL synchronization?
Not necessarily. Identity synchronization and Exchange GAL visibility are different concerns. Confirm the target recipient objects are mail-enabled and visible as required.
Why can I find a coworker in Outlook but not in my iPhone contacts?
Outlook directory lookup does not automatically create a native smartphone contact. Organizations that require saved company contacts on mobile devices need a suitable contact synchronization workflow. [4]
When should IT teams configure contact synchronization during migration?
Audit the existing process before migration, test the destination configuration with pilot users, and validate the final synchronization setup after cutover. Avoid switching production jobs without verifying source data, permissions, and duplicate handling.
Is CiraSync an Office 365 tenant migration tool?
No. CiraSync focuses on ongoing Microsoft 365 contact synchronization. Mailbox and workload migration require separate supported procedures or tools. [6]
