If users report missing content after a MigrationWiz migration, the problem may not be the migration itself. It may be mailbox sizing on the destination. A common scenario is the destination quota being smaller than the source mailbox at the time the job starts.

MigrationWiz can migrate individual items only up to the destination Exchange Online message size limit. In Exchange Online, the default maximum message size is 150MB. Any source item above that limit fails at the item level, is recorded in the error report, and is skipped while the job continues.

More often, the issue is total mailbox quota rather than a single oversized message. For example, if the destination mailbox has a 50GB quota and the source mailbox contains 55GB, MigrationWiz will migrate items until the destination fills. Once the quota is exhausted, item-level quota exceeded errors begin and the job ends up partially complete.

To avoid finding this out after the fact, check mailbox sizes before migration. Pull source mailbox statistics in the Exchange Admin Center or via PowerShell using Get-EXOMailboxStatistics, compare those numbers to destination quota assignments, and then either increase destination quotas or reduce source mailbox content before running the batch.

Are you comparing source mailbox sizes against destination quota assignments before submitting MigrationWiz jobs?

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