[Exim] Problem with Envelope-to header

in a transport I try:
command = /usr/sbin/inflex-exim $h_envelope-to: $sender_address
$received_for
but I didn't get a value for the Envelope-to: header.
Could anyone give me an idea.
virus_scan:
    driver = pipe
    command = /usr/sbin/inflex-exim $sender_host_name $h_envelope-to:
$sender_address $sender_address
    envelope_to_add # I've tried it also without
    user = root
    group = root
    path = "/bin:/sbin:/usr/bin:/usr/sbin
Regards
Wolfgang