Wanted to open this up for a real discussion rather than just marketing pages, since most vendor sites make every tool sound flawless until you're actually in the middle of a cutover weekend. We recently went through an M365 tenant-to-tenant migration following a company acquisition, and a few lessons stood out that I think are worth sharing.
Native Microsoft options work fine for basic scenarios, but once you factor in shared mailboxes, deep folder hierarchies, and users who need minimal downtime, things get trickier fast. We ended up testing a few third-party tools, and the one that gave us the least trouble in practice was this
DRS Softech Tenant to Tenant Migration Tool. It handled our mailbox, calendar, and contacts migration without the folder-flattening issues we ran into with another tool during testing, and support was responsive when we hit a licensing mismatch mid-project.
One real pain point we didn't anticipate was user communication — even with a solid tool, if end users aren't warned about temporary sync delays or Outlook re-authentication, you'll get a flood of helpdesk tickets regardless of how good the backend migration is. Delta migration support helped reduce our final cutover window significantly compared to a full one-shot migration.
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