$mbx = get-mailbox -ResultSize unlimited

$hash = @{}

$mbx | % {
    $addresses = $_.EmailAddresses | ? {($_ -like "*smtp*")}
    
    foreach($address in $addresses)
    {
        $pos = $address.IndexOf("@")
        $suffix = $address.Substring($pos+1)

        if($suffix)
        {
            $hash[$suffix]++
        }

        else
        {
            $hash["Blank"]++
        }
    }
}

$hash.GetEnumerator() | Sort -Property Value -Descending | Out-File "SMTPDomains.txt"