We're preparing for a tenant consolidation following a business acquisition, and the mailbox migration is turning out to be one of the more complicated parts of the project. We have users spread across different departments, and many of them have large mailboxes containing years of customer conversations, internal discussions, attachments, calendars, and contacts.

The first thing we're trying to establish is how to Migrate Emails from Office 365 Tenant to Tenant without creating unnecessary downtime. We don't want employees to stop using email for an extended period just because the IT team is transferring historical data. At the same time, we need confidence that the destination mailboxes contain everything required before the source environment is retired.

One area we're testing is mailbox mapping. Some users will retain their existing names, while others will receive new addresses under the acquiring company's domain. We're creating a source-to-destination mapping sheet and validating it before starting any large migration batch. This should reduce the possibility of accidentally transferring one employee's mailbox into another user's account.

We're also looking at how to divide the workload. Instead of migrating everyone together, we're considering smaller batches based on department and mailbox size. A pilot group will be migrated first so we can compare email counts, folder structures, attachments, contacts, and calendars. Once the results look correct, we'll gradually increase the batch size.

The final cutover is another concern. Employees will continue sending and receiving messages while the initial migration is running, so there will inevitably be new data after the first transfer. A final incremental migration could help capture those changes immediately before users start working exclusively from the new tenant.

Security and authentication are also being reviewed. We don't want to rely on shortcuts just to finish the project faster. Both tenants need to be configured correctly, and the administrators responsible for the migration need appropriate access without unnecessarily exposing user credentials.

During the research phase, the DRS Softech O365 Tenant to Tenant Migration Tool came up as one of the options for handling this type of project. Its listed features include mailbox mapping, Modern Authentication, incremental migration, filtering, deduplication, migration reports, and support for SharePoint and OneDrive in addition to Exchange Online.

I'm curious how other IT teams have approached similar tenant mergers. Did you run a complete pilot before starting production migration? How did you decide the batch size, and what checks did you perform before decommissioning the source tenant? Any practical advice about validation, DNS changes, or user communication would be helpful.