Preparing Users for Orchestrated Migration: MailUsers, Licensing Order, and Common Failure Patterns

Orchestrated migration requires that both source user objects and target user objects exist. Your most important goal is to create target users in the right state so identity mapping and migration can run without failures.

Non-negotiable rule: do not provision target mailboxes or OneDrive sites early

Microsoft emphasizes an order of operations: complete identity mapping before assigning Exchange or OneDrive licenses to target users. If you license target users too early, they may provision mailboxes, breaking the required MailUser state.

Create target MailUsers (MEUs) in Exchange Online

A MailUser object (Mail-Enabled User / MEU) must exist in the target tenant for each migrating source user. CTIM stamps ExchangeGuid and other attributes later, so you do not need to pre-populate everything up front.

Connect-ExchangeOnline

New-MailUser -PrimarySmtpAddress username@targettenant.com -MicrosoftOnlineServicesID username@targettenant.com -ExternalEmailAddress username@sourcetenant.com -Name "First Last"

Two common failure patterns to warn about:

·       Non-accepted domains: target MailUsers must not have PrimarySmtpAddress or EmailAddresses from non-accepted domains, or the process can fail for that user.

·       Licensing too early: if you assign E3/E5-type licenses before identity mapping stamps ExchangeGuid, the target user may become a mailbox user instead of a MailUser, causing migration failure.

OneDrive readiness: prevent target OneDrive provisioning

For OneDrive migrations (orchestrated or dedicated), Microsoft explicitly warns that target OneDrive sites should not be created before or during the migration. If a target OneDrive site already exists, the migration fails because it cannot overwrite an existing site.

Scoping discipline: one user, one active batch

Plan user waves carefully. A user can only be in one active orchestrator batch at a time, and each batch is limited to 100 users. Use scope groups and pilot rings to reduce operational risk.

Source links (Microsoft Learn)

·       https://learn.microsoft.com/en-us/microsoft-365/enterprise/migration-orchestrator-4-user-prep?view=o365-worldwide

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