I’m preparing for an Exchange Server to Microsoft 365 migration and have started looking into the archived mailbox data separately. There are several years of emails in the archives, so simply moving the active mailboxes isn’t enough. The historical messages also need to remain available after the migration.

For administrators who have done this before, what checks should be completed before you Migrate Archive Mailboxes to Microsoft 365 ? I’m currently considering the archive size, Exchange configuration, Microsoft 365 account setup, permissions, retention policies, and whether the destination archive is properly configured.

Another thing I’m trying to understand is how to handle large archive datasets. With multiple users, manually checking each mailbox and dealing with duplicate or already migrated messages could make the project unnecessarily complicated.

While researching different approaches, I came across the SysInfo Office 365 to PST Export Tool. It offers options such as date filtering, duplicate removal, user mapping, and skipping previously migrated data. The migration reports could also help administrators verify what was transferred after the process is complete.

I’d like to know how others have approached this. Did you use Microsoft’s native Exchange migration options, a third-party tool, or both? If you used a third-party application, what factors did you consider before selecting it? Any practical advice regarding permissions, archive configuration, or post-migration verification would also be helpful.