Best Practices for Secure Yahoo Mail to Office 365 Migration

One thing I'd add from experience is to spend a little extra time preparing the source mailboxes before starting the migration. Microsoft's IMAP migration works well for Yahoo Mail, but make sure all target Microsoft 365 mailboxes are already created and licensed, since the migration process copies mail into existing mailboxes rather than creating them for you.

Also, Yahoo has moved toward modern authentication, so if you're connecting through IMAP, verify that the account supports the required authentication method before you begin. This avoids a lot of the "authentication failed" issues that people run into during migration.

For a handful of mailboxes, the native Microsoft migration tools are usually sufficient. If you're dealing with a larger number of Yahoo accounts or want additional controls such as mailbox mapping, migration filters, detailed logs, or the ability to retry failed items without starting over, there are third-party tools available as well. I came across the BitRecover Yahoo Mail to Office 365 Migration Tool while comparing different approaches, and it seems to cover those scenarios.

Regardless of the method you choose, I'd recommend running a small pilot migration first with one or two mailboxes, validating folder structure and message counts, and then proceeding with the remaining users. It makes the final cutover much smoother.