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Unfortunately it is not possible for you to easily determine the number of items in your mailbox. If you do have your mailbox pre-staged you’ll only know because you have historical email items appearing in your Exchange Online mailbox prior to the end of the week. Should this be the case, please follow the points to note section at the end of this page.
Here is the schedule of migration dates for the remaining departments: Large Department Migration Dates .
Larger Sized Mailboxes
If you already have Exchange Online and your mailbox has more than 60,000 items, your migration will happen in 2 parts.
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