Cross-Tenant Migration Start with MTO: From Day-1 Collaboration to Migration Readiness

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In many tenant-to-tenant programs, the business need arrives before the migration plan: "We need to collaborate now." Microsoft's guidance reflects this reality by recommending that organizations establish a Multitenant Organization (MTO) for day-1 collaboration and then transition into migration readiness in a controlled way.

Day-1 collaboration: what MTO enables

MTO enables immediate cross-tenant collaboration without changing user credentials. Typical steps include:

·       Create the MTO in the Microsoft admin center.

·       Configure trust relationships and policies.

·       Sync users as B2B external members for cross-tenant collaboration.

·       Enable collaboration features (Teams, file sharing, profile cards).

Moving from MTO to migration readiness: the key concept

The transition from MTO collaboration to migration readiness is about narrowing scope, hardening controls, and preparing identity and mail objects so migrations land cleanly in the target tenant.

Premigration preparation checklist (core items)

·       Define migration scope using mail-enabled security groups in MailboxMovePublishedScope (maximum 5,000 objects per scope). Avoid nested groups and ensure a user is not in more than one scope.

·       Configure organization relationship for mailbox moves (MailboxMoveEnabled = $True).

·       Install required PowerShell modules (ExchangeOnlineManagement, Microsoft.Graph, Microsoft.Graph.Beta, CrossTenantIdentityMapping with -AllowPrerelease).

·       Ensure target tenant MailUser objects exist for each migrating user (accepted domains only).

Example: create and populate a migration scope group

New-DistributionGroup -Type Security -Name "[MigrationScopeGroupName]" -DisplayName "Migration Scope for [Project or Batch Name]"

Add-DistributionGroupMember -Identity "[MigrationScopeGroupName]" -Member <UserOrGroup>

Get-DistributionGroupMember -Identity "[MigrationScopeGroupName]" | Select Name, PrimarySmtpAddress

Operational tip: treat the MTO-to-migration transition as a change event

Migration readiness is not only a technical configuration exercise. It changes how users appear in the target tenant, how mail routing works, and when users should begin using the target identity. Plan communications and support readiness as carefully as you plan the cutover itself.

Source links (Microsoft Learn)

·       https://learn.microsoft.com/en-us/microsoft-365/enterprise/cross-tenant-multitenant-migration?view=o365-worldwide

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