Are you launching 300 mailbox jobs at the same time in MigrationWiz and watching EWS throttling grind the entire batch down?

Microsoft throttles Exchange Online at the tenant level, not per mailbox. When 300 MigrationWiz jobs start simultaneously, they all compete for the same tenant-wide EWS resource budget. The result is that everything slows down instead of a smaller set of jobs running at full speed.

A batching strategy that works is to split the population into groups of 50 to 75, start each batch 30 to 45 minutes apart, and watch throughput on batch one before triggering batch two. If service protection throttling shows up within the first two hours on the first batch, reduce the concurrent job count before the next group begins.

MigrationWiz includes a job scheduler inside each project. Use it to set staggered start times across batches rather than one global start time for everyone.

What batch size and stagger interval are you using for your MigrationWiz mailbox job submissions?

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