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Prepare sub-domain for Microsoft 365-to-Microsoft 365 migration

Because only one Microsoft 365 tenant can validate access to a DNS namespace at a time, any M365-to-M365 migration that preserves users’ original email addresses will need to transfer domains between tenants. Further, you may need to either create a temporary DNS namespace to preserve access to the old tenant’s data after migration, or utilize Microsoft’s included onmicrosoft.com domain as a sub-domain to facilitate domain registration in the target tenant.

When adding an alternative domain you’ll need to register and validate it as a secondary domain in the source tenant and assign aliases for all accounts that need access. If users need to receive mail at these addresses you’ll also need to update MX records to configure delivery. It is recommended this is performed prior to migration. See Microsoft’s documentation about adding a domain.

After you’ve added the alternative domain in the source tenant (or decided to utilize the onmicrosoft.com domain) you’ll want to make it the primary domain for user accounts in the source tenant. When performing a Direct Transition (all users transition to the new tenant at the same time) this will typically be after all migrations are complete.

To remove the domain from the source tenant you’ll need to delete or rename all accounts (setting the alternative domain as their sole domain, including groups, rooms, etc.) to purge references to the transferring domain. See Microsoft’s documentaiton about removing a domain.

Once the original namespace is delisted from the source tenant you should be able to validate it as a secondary domain in the target tenant, which will allow you set the primary email address or create an alias with the source domain in the target tenant.

Sequence of events

Below represents the recommended sequence of events related to domain registration when migrating between Microsoft 365 tenants where the target tenant will use a new domain, but you want to provision email aliases in the new tenant with the source domain:

Prior to Cutover:

• Register a new DNS domain on target tenant
• Provision user accounts in target tenant with the new domain
• Provision a temporary DNS alias for accounts on source tenant (using a newly registered sub-domain or by using the onmicrosoft.com domain)
• Ensure translation table in TMC is properly configured
• Perform a Stage 1 migration of email to the target accounts

At Cutover time

• Instruct users to make no more changes in the source environment
• Perform Stage 2 migration (filtered run of recent mail plus full migration of calendars, contacts, tasks, as necessary)
• Remove any addresses and aliases that use the migrating DNS namespace from source accounts
• Remove the migrating DNS domain from the source tenant
• Validate DNS domain in the target tenant
• Assign SMTP addresses and aliases to migrated accounts in the target tenant
• Set primary SMTP address to production address for all users