Our IT department is preparing for an acquisition where two Microsoft 365 environments will eventually need to operate under a single tenant. The number of users is significant, but the bigger challenge is coordinating identity, mailboxes, domains, security settings, and user access without interrupting normal business operations.

The immediate requirement is to Migrate Email Between Two Microsoft 365 Tenants while keeping existing users productive throughout the transition. We have several years of mailbox data, including custom folders, attachments, contacts, calendars, and important customer correspondence. Before moving anything, we're creating an inventory of the source environment so that we know exactly what needs to be transferred and what can be archived separately.

One lesson we're learning during the planning stage is that mailbox mapping needs to be finalized early. The source and destination tenants won't have identical user structures, and some employees will receive new addresses after the acquisition. We're therefore preparing a mapping document and checking it against the HR/user directory before any migration batch is started.

We're also considering a pilot migration. Rather than selecting only small mailboxes, we want the test group to include different mailbox sizes and users with complicated folder structures. After the pilot, we'll verify email counts, attachments, folders, contacts, and calendars. The results should help us determine realistic migration times and identify any permissions or authentication issues.

Another topic is incremental migration. Employees cannot stop using email while the project is running, so data will continue changing after the initial transfer. A second migration pass could capture new items before the final cutover, reducing the amount of work required during the maintenance window.

The project also includes OneDrive and SharePoint, which means our migration plan has to consider more than Exchange Online. We need to understand how user mapping will affect files, sites, permissions, and access after the destination accounts are activated.

While comparing solutions for the project, I found the DRS Softech O365 Tenant to Tenant Migration Tool. Its documented capabilities include Exchange Online, SharePoint, and OneDrive migration, source-to-destination mapping, Modern Authentication, incremental migration, filtering, deduplication, progress monitoring, and detailed reports.

For teams that have completed a Microsoft 365 migration after an acquisition, what did you prioritize during the planning phase? Did you create a detailed inventory first, perform a pilot, or start by configuring the destination tenant? I'm particularly interested in lessons that can prevent problems during the production migration.