I need to transfer a domain from one Microsoft 365 tenant to another, and I’m trying to understand the correct order of steps before making any changes. The domain is already being used for business email, so my biggest concern is avoiding missed messages or login problems while the transfer is taking place.

From what I understand, the process starts with preparing both tenants. The source tenant should be reviewed for users, groups, mailboxes, aliases, proxy addresses, and UPNs connected to the domain. On the destination side, temporary onmicrosoft.com accounts can be prepared, licenses assigned, and the required users and resources planned before the actual domain change.

DNS and mail routing also seem to be important. Reducing the MX-record TTL before the migration can help when DNS changes are required. During the transition, temporary addresses and mail-routing arrangements may be needed so that incoming messages are not simply left without a valid destination.

I’m also trying to separate the domain transfer from the actual data migration. Moving the domain does not by itself mean that mailbox data, OneDrive files, or SharePoint content has been transferred. For a larger project, I would first want to protect the existing data and then migrate domain from one O365 tenant to another as part of a properly planned tenant-to-tenant process.

One approach I found involves using the SysInfo Office 365 Tenant to Tenant Migration Tool. According to the source I was reading, it can be used to connect the source and destination environments, select and map mailboxes, apply filters, and perform the migration. It also describes incremental migration and deduplication options, along with preservation of folder structure and metadata.

Once the domain is no longer referenced in the source tenant, it can be removed and then added to the destination tenant for verification. Afterward, the primary SMTP addresses and UPNs can be updated, MX records changed, and mail flow tested. Shared mailboxes, groups, calendars, OneDrive or SharePoint links, and applications should also be checked after the transition.

For administrators who have completed an Office 365 migrate domain from one tenant to another  project, how did you handle the period between removing the domain from the source and verifying it in the destination? Did you use temporary addresses and staged mail routing, and what checks did you perform before considering the transfer complete?