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$filter = "(proxyAddresses=SMTP:foo@contoso.com)"

$gcs = [System.DirectoryServices.ActiveDirectory.Forest]::GetCurrentForest().GlobalCatalogs
$gcs | % {
    $searcher = $_.GetDirectorySearcher()
    $searcher.Filter = $filter
    $results = $searcher.FindAll()
    if ($results.Count -gt 0)
    {
        $results