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$path = '\\server\path\'
$mailboxes = get-mailbox somemailboxfilter
$mailboxes | % {
    $folders = Get-MailboxFolderStatistics $_.Alias -archive
    $folders = $folders | ? {$_.Name -ne "Top of Information Store"}
    $excludeFolders = $folders | % { $_.FolderPath.Substring(1) }
    New-MailboxExportRequest -Mailbox $_.Alias -ExcludeFolders $excludeFolders -ExcludeDumpster -IsArchive -FilePath ($path + $_.Alias + '.pst')
}