After a merger or acquisition, organizations often operate multiple Microsoft 365 tenants while integrating their people, systems and directories. During this coexistence period, employees need to find colleagues across company boundaries, even when their accounts remain in separate tenants. Microsoft Entra cross-tenant synchronization can provision users into another tenant and support GAL visibility when the required address-list attributes and Exchange recipient settings are correctly configured. However, making employees searchable in Outlook does not automatically synchronize their contact information to smartphones.
That split matters because a newly combined company rarely runs on one tenant from day one. Domains, mailboxes and identities usually stay in separate tenants for months while the integration team plans consolidation. During that time, employees expect to find their new colleagues in Outlook, Teams and their phone’s Contacts app as if they already worked for one company.
This guide explains why GALs do not merge on their own, what Microsoft’s native tools do and do not cover, the decisions to make before you sync anything, and a phased approach from day one through consolidation.
Key takeaways
- Microsoft now offers native cross-tenant user sync. Cross-tenant synchronization can make synced users discoverable in the target tenant’s GAL, and it underpins Microsoft 365 multitenant organizations.
- It syncs users and security groups, not contacts. Microsoft states that contacts and devices are not supported, and distribution lists are not created in the target tenant.
- A searchable GAL is not a phone directory. Synced colleagues can be found in Outlook, but they do not appear by name on incoming calls unless they are delivered to the phone’s contacts.
- Pre-close sync needs a legal check. Microsoft intends cross-tenant synchronization for use within one organization and notes extra compliance duties, including GDPR, when it is used across organizations.
- Plan for three phases. Day one, coexistence and consolidation each need a different contact strategy.
Why GALs do not merge on their own after an acquisition
Each Microsoft 365 tenant has its own Microsoft Entra directory and its own Exchange Online GAL. Nothing in one tenant appears in another unless someone puts it there. Three constraints keep the tenants apart longer than most business leaders expect.
- A domain belongs to one tenant at a time. Microsoft’s cross-tenant migration guidance states that domains are associated with only one Microsoft Entra ID or Exchange Online tenant (Microsoft Learn). The acquired company’s email domain cannot move until its mailboxes are ready to move with it.
- Consolidation is a project, not a switch. Mailbox moves require licensing, identity preparation and batch planning, so many organizations run two or more tenants side by side for months.
- Some organizations never consolidate. Regulatory separation, regional data requirements or a planned divestiture can make a long-term multi-tenant setup the intended end state.
During that coexistence period, the GAL is the first place people notice the gap. A sales rep cannot find the acquired company’s account manager in Outlook, a new manager cannot address their team, and calls from colleagues arrive as unknown numbers.
Microsoft’s native options for cross-tenant GALs
Microsoft offers three native building blocks. They solve different parts of the problem and are often combined.
Microsoft Entra cross-tenant synchronization
Microsoft Entra cross-tenant synchronization provisions supported B2B collaboration users from a source tenant into a target tenant. In March 2026, Microsoft introduced cross-tenant security-group synchronization in preview for supported same-cloud environments. This functionality supports eligible security groups but does not create distribution lists, mail-enabled security groups or Microsoft 365 groups in the target tenant. Microsoft documents the following capabilities and requirements:
- It is a push from the source tenant, where admins choose the users in scope and the attribute mappings.
- It syncs only internal members of the source tenant, not that tenant’s own external users.
- It runs on a fixed cycle that starts every 40 minutes, with a longer first run.
- It can enable people search in Microsoft 365. The showInAddressList attribute is set to True by default in the attribute mappings.
- Each synced user needs a Microsoft Entra ID P1 license in the source tenant.
Once users are synced, Microsoft says people in the target tenant can find them in Outlook search and the To line, and open a people card (Microsoft Learn). Since late 2025, users hidden from the GAL in the source tenant also stay hidden in the target (Microsoft message center, MC1187400).
Microsoft 365 multitenant organization
A multitenant organization groups the tenants your company owns into one boundary, with improved collaboration in Teams and Viva Engage (Microsoft Learn). It relies on provisioning member users across tenants, typically through cross-tenant synchronization. Microsoft 365 multitenant organizations support up to 100 tenants in the standard configuration, and each tenant can belong to only one multitenant organization. Microsoft Entra ID P1 or higher licensing is required in participating tenants. Cross-tenant user synchronization requires an Entra ID P1 license for each synchronized user in the source tenant, while group synchronization has additional licensing requirements. Administrators should verify licensing and supported tenant configurations before implementation.
Mail contacts (traditional GAL sync)
The older approach represents users from another tenant as mail contacts. Microsoft’s Entra architecture guidance explains the trade-offs (Microsoft Learn):
- Mail contacts appear in the GAL without granting access, because they are not security principals.
- Microsoft Graph has no API for managing Exchange mail contacts, so admins need Exchange Online PowerShell or the Exchange admin center to create and manage them.
- A mail contact and a mail-enabled external user cannot exist in the same tenant with the same email address.
How the options compare
| Capability | Cross-tenant synchronization | Multitenant organization | Mail contacts |
| What it creates | B2B users and security groups | Tenant group that uses B2B member users | Contact objects in the GAL |
| Appears in the GAL | Yes, when showInAddressList is True | Yes, through synced member users | Yes |
| Enables cross-tenant resource access | Provisions B2B identities that can be granted access according to applicable policies and permissions | Supports collaboration through provisioned B2B users and configured access policies | No, mail contacts are not sign-in identities |
| Syncs external contacts and distribution lists | No | No | Must be created separately |
| Puts colleagues in phone contacts | No | No | No |
| Licensing noted by Microsoft | Entra ID P1 per synced user in the source | Entra ID P1 | None stated |
Microsoft recommends B2B collaboration rather than traditional GAL synchronization for representing users who need to appear in the GAL. Mail contacts remain useful for directory-only visibility and for external contacts that cross-tenant sync does not cover.
What native cross-tenant sync does not solve
When configured correctly, cross-tenant synchronization can make users from another tenant discoverable in Microsoft 365. However, identity provisioning does not automatically guarantee GAL visibility or synchronize contacts to employee phones. Six gaps usually remain for M&A teams.
Phones still show unknown numbers
A user who is searchable in the GAL is not a saved contact on anyone’s phone. Outlook and an iPhone’s Exchange account can look the person up, but the phone’s Contacts app, which drives caller ID, only shows contacts synced to it. For the people who call each other across the two companies, the directory still has to be delivered to their phones. Our GAL sync guide explains this directory-to-phone gap.
External contacts and distribution lists stay behind
Microsoft states that cross-tenant synchronization supports users and security groups, while devices and contacts are not supported. It does not create distribution groups, mail-enabled security groups or Microsoft 365 groups in the target tenant (Microsoft Learn). Vendor, partner and customer contacts in either company’s GAL need a separate plan.
Profiles are thinner than in the home tenant
Microsoft Entra cross-tenant synchronization does not support synchronizing user profile photos between tenants. As a result, synchronized users may appear without their profile pictures in Microsoft 365 people-search experiences unless those photos are provided through another supported method. Other profile information available in the target tenant depends on the attributes synchronized and the Microsoft 365 application being used.
Conflicts between mail contacts and synced users
If a GAL sync tool has already created mail contacts for the other company’s employees, those records collide with cross-tenant sync. Microsoft notes that a mail contact and a mail-enabled external user cannot share an email address in the same tenant, and inviting a B2B member when a matching mail contact exists returns a proxy address conflict (Microsoft Learn). Decide which mechanism represents each person before turning either one on.
Hybrid tenants may not see the same GAL on-premises
Microsoft lists write-back to on-premises Windows Server Active Directory as something cross-tenant sync does not provide. If the target company still hosts mailboxes on Exchange Server, users there may not see the synced colleagues that cloud mailboxes see.
Pre-close sharing needs legal review
Microsoft says cross-tenant synchronization is intended for use within an organization. Using it across organizations can add compliance responsibilities, including under the GDPR, and Microsoft advises consulting legal or compliance teams first. Before a deal closes, the two companies are still separate organizations, so get sign-off on what directory data can be shared and with whom.
Decisions to make before unifying GALs
Settle these eight questions with HR, legal and both IT teams before syncing anything. Changing them later means cleaning up thousands of directory objects.
Before enabling cross-tenant synchronization or a GAL-to-GAL synchronization tool, audit the target tenant for existing mail contacts, B2B users and duplicate email addresses. Confirm which system will manage each object type. Conflicting objects can cause provisioning failures, duplicate directory entries or unexpected changes to address list visibility. Test the synchronization configuration with a limited group before expanding it across the organization.
Important deprovisioning consideration: Removing users from cross-tenant synchronization scope can trigger soft deletion of their corresponding target tenant objects when deletion actions are enabled. Before changing synchronization scope, IT teams should verify object dependencies, review the provisioning configuration and coordinate with the identity and migration teams to prevent unintended account disruption.
| Decision | Questions to answer | Why it matters |
| Scope | Which employees from each company should appear in the other GAL? All staff, or only certain departments? | Cross-tenant sync and GAL sync tools both work from a defined scope. Too broad exposes data; too narrow frustrates users. |
| Direction | One-way from the acquired company into the parent, or both ways? | Cross-tenant sync runs one direction per configuration, so mutual visibility needs one job in each direction. |
| Object types | Users only, or also shared mailboxes, rooms, external contacts and distribution lists? | Native user sync does not cover contacts or distribution lists. |
| Source of authority | Which tenant owns each person’s attributes during coexistence? | Microsoft treats the source tenant as authoritative; edits in the target are overwritten when the source changes. |
| Naming | Will display names show the company, for example a suffix or the company attribute? | Two people with the same name in a merged GAL cause misaddressed email. |
| Representation | Will each person appear as a synced B2B user or as a mail contact? | The two cannot share an email address in the same tenant. |
| Phones | Which teams need the other company’s people as phone contacts? | GAL visibility alone does not provide caller ID. |
| End state | Consolidate into one tenant, or stay multi-tenant long term? | This decides whether sync is temporary scaffolding or permanent infrastructure. |
The source-of-authority point comes from Microsoft’s FAQ: cross-tenant synchronization treats the source tenant’s values as authoritative, and changes made in the target persist only until the user changes in the source (Microsoft Learn).
A phased approach: from deal close to consolidation
Each phase of an integration needs a different contact strategy. Work through them in order.
Phase 1: Before the deal closes
Get legal and compliance approval for any directory data shared between the companies.
Inventory each tenant: user counts, mail contacts, distribution lists, shared mailboxes, rooms and any existing GAL sync tools.
Check Microsoft Entra ID P1 licensing for users you plan to sync.
Agree on the decisions in the table above.
Phase 2: Day one
Turn on cross-tenant synchronization for the users who need to find each other first, typically leadership, sales, finance and IT.
Confirm showInAddressList is set so synced users appear in each GAL.
Push a short list of essential contacts to employees’ phones, such as executives, help desks and key site contacts, so calls show names from the first week.
Phase 3: Coexistence
Expand the sync scope in batches and monitor provisioning logs.
Create mail contacts or distribution lists that cross-tenant sync does not cover, avoiding duplicates of synced users.
Deliver the combined directory to phones for the teams that call across companies.
If the companies plan to stay separate long term, consider a multitenant organization for the Teams and Viva Engage experience.
Phase 4: Consolidation
Plan mailbox consolidation using a supported migration method, such as Microsoft’s cross-tenant mailbox migration capability, where the environment meets its prerequisites. This process is separate from Microsoft Entra cross-tenant synchronization and requires additional planning for licensing, target identities, mailbox attributes, migration batches and domain cutover. Validate the migration method and its limitations before moving production mailboxes. Microsoft notes that cross-tenant synchronization is not a migration tool, because synced users still authenticate in their source tenant.
Pre-create external contacts and distribution lists in the target tenant, as Microsoft’s FastTrack guidance requires (Microsoft Learn).
Before removing migrated users from cross-tenant synchronization, verify how their target tenant identities and mailbox objects will be managed after migration. Removing a user from synchronization scope can trigger soft deletion of the corresponding target object, depending on the provisioning configuration. Coordinate identity cutover, mailbox migration and synchronization deprovisioning, and test the process before applying it to production users.
Clean up the source tenant. Microsoft notes the source mailbox becomes a mail user after migration, which can later be removed or converted to a mail contact (Microsoft Learn).
Rebuild phone contact delivery for migrated users, since their mail accounts and contact sources change.
How CiraSync supports multi-tenant contact sync
CiraSync and CiraHub support different parts of enterprise contact synchronization. CiraHub provides multi-way synchronization between connected directories and other supported contact sources, while CiraSync helps deliver company contacts to supported employee devices. Together, these capabilities can help IT teams maintain contact availability across multiple Microsoft 365 tenants during mergers, acquisitions and ongoing coexistence.
Unifying company directories during M&A with CiraHub. When two organizations operate separate Microsoft 365 tenants, CiraHub can synchronize supported contact information between connected directories. IT teams can configure mappings, filters and synchronization rules to manage which contact records are shared across the organizations during coexistence.
Maintaining consistent contact information across tenants. During a merger, employee details, phone numbers and organizational information may change frequently. CiraHub’s multi-way synchronization capabilities help keep supported contact records aligned across connected systems, reducing the need for manual directory updates while the tenants remain separate.
Making cross-company contacts available on employee phones. Once the relevant contact information is available, CiraSync Cloud can synchronize selected company contacts into targeted Microsoft 365 mailbox Contacts folders. For supported managed-device environments, Mobile Direct provides an alternative CardDAV-based delivery method. This helps employees access colleagues’ contact details on their smartphones even while the organizations continue operating separate tenants.
If you also run Microsoft Entra cross-tenant synchronization, decide which tool represents each person in each GAL, so the same employee is not created twice.
In a published example, a $100B+ alternative investment firm with more than 2,000 employees across two dozen offices needed corporate contacts to reach a mailbox in a second Microsoft 365 tenant. CiraSync reports that multi-way sync was live within weeks, without custom engineering (case study).
Plan your multi-tenant directory
If you are preparing for a close or already running two tenants, talk to a CiraSync specialist about scoping GAL sync and phone contacts for each phase, or start a free trial with a pilot group.
Frequently asked questions
Can Microsoft 365 sync the GAL between two tenants natively?
Partly. Microsoft Entra cross-tenant synchronization can provision users from one tenant into another and make them discoverable in the target tenant’s GAL. It does not sync contacts or create distribution lists, so those still need a separate process.
What is the difference between cross-tenant synchronization and a multitenant organization?
Cross-tenant synchronization is the engine that provisions users and security groups between a source and a target tenant. A multitenant organization is a grouping of tenants your company owns, which improves Teams and Viva Engage collaboration and typically relies on cross-tenant synchronization to provision member users.
Does cross-tenant synchronization put contacts on employees’ phones?
No. It creates B2B users in the target directory, which Outlook can search. Phones show names on incoming calls only for contacts synced to the device, so the directory still has to be delivered to phones.
How often does cross-tenant synchronization update?
Microsoft says the sync interval is fixed to start every 40 minutes. The first cycle takes longer, depending on how many users are in scope.
Can we sync GALs before the acquisition closes?
It is technically possible, but Microsoft intends cross-tenant synchronization for use within an organization and notes added compliance responsibilities, including GDPR, when used across organizations. Get legal approval before sharing directory data with a company you do not yet own.
Should we use mail contacts or synced users?
Use synced users when people need access to resources in the other tenant. Mail contacts suit directory-only visibility and external contacts. Avoid both for the same person, because Microsoft notes a mail contact and a mail-enabled external user cannot share an email address in one tenant.
Is cross-tenant synchronization a migration tool?
No. Microsoft states that synced users still authenticate in their source tenant, and that tenant migrations also require moving data. Use cross-tenant mailbox migration for consolidation.
What licenses does cross-tenant synchronization need?
For cross-tenant user synchronization within the same Microsoft cloud, each synchronized user requires a Microsoft Entra ID P1 license in the source tenant. Synchronizing security groups requires Microsoft Entra ID Governance or Microsoft Entra Suite licensing. Cross-cloud synchronization also requires Microsoft Entra ID Governance or Microsoft Entra Suite licensing. Target tenant licenses are not required specifically for the synchronization process, although additional licenses may be needed for other Microsoft 365 services or features.
