$helo_name
$sender
$sender_????
$recipient
$recipient_???
$client    <---   name[address]
$client_???
$reverse_client_name   <---  address->name lookup, or "unknown"

${name?text}
    Expands to `text' if $name is not empty.
${name:text}
    Expands to `text' if $name is empty.