How to get Mailbox list in batches of 100 using -ResultSize

Question

How to get Mailbox list in batches of 100 using -ResultSize

Yash Vora 20

I have to fetch Exchange Mailboxes in batches of 100 (there are total 1m+ mailboxes in organization).

I am using the following powershell command:

Get-EXOCASMailbox -ResultSize Unlimited

This gives me all the mailboxes which is taking long time. I can process the result in batches of 100 so I hit this command: Get-EXOCASMailbox -ResultSize 100

Seems to work fine but I'm stuck on how to bring the next 100 mailboxes.

PS: You can assume that the mailboxes are static and no changes will be made on them for indefinite time...

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Answer accepted by question author

Anonymous

Hi Yash Vora

In theory, -ResultSize Unlimited should return all mailboxes in your organization.

However, since your environment contains more than 1 million mailboxes, using -ResultSize Unlimited alone may not be practical, as it can be impacted by disk capacity and throughput limitations.

With that in mind, I would recommend applying additional filters based on specific attributes such as:

Department
RecipientTypeDetails

This approach can help narrow down the results and make the query more manageable. You can then export each filtered result into separate CSV files for easier review and future reference.

For example, you may add the following after your PowerShell command Get-EXOCASMailbox:

| Export-Csv -Path "C:\Temp\EXOCASMailbox.csv" -NoTypeInformation -Encoding UTF

For more detailed guidance, you may refer to: Export-Csv (Microsoft.PowerShell.Utility) - PowerShell | Microsoft Learn

I hope this information provides helpful direction and adds further clarity.

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2 additional answers

Your answer

Answer 1

Vasil Michev 128.1K MVP Volunteer Moderator

The cmdlets themselves do not support pagination, but the underlying API does. So if for some reason you absolutely must use pagination, I'd suggest you take a look at https://michev.info/blog/post/5822/all-you-need-to-know-about-exchange-online-admin-api-or-how-to-run-cmdlets-without-powershell

In the request, set the Prefer header with value of odata.maxpagesize=100. And remember to follow the @odata.nextLink URLs :)

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Answer 2

-ResultSize limits how many objects the cmdlet returns, but it does not provide paging or a way to ask for “the next 100” directly. Each call starts again from the beginning of the result set.

To process in batches of 100, retrieve all mailboxes once, then batch them in PowerShell:

# Get all CAS mailboxes once
$mailboxes = Get-EXOCASMailbox -ResultSize Unlimited

# Process in batches of 100
$mailboxes | ForEach-Object -Begin {
    $batch = @()
} -Process {
    $batch += $_
    if ($batch.Count -eq 100) {
        # Do work on this batch of 100
        $batch | <# your processing here #>

        # reset batch
        $batch = @()
    }
} -End {
    # process any remaining items (< 100)
    if ($batch.Count -gt 0) {
        $batch | <# your processing here #>
    }
}

If the goal is to reduce the time and data volume per call, use filters or property sets to narrow what Get-EXOCASMailbox returns, but paging still must be implemented client-side as above.

For mailbox objects (not CAS settings), the same batching pattern applies with Get-EXOMailbox -ResultSize Unlimited.

References: