But the variable $RECIPIENT contains normally the recipient of the email, so the script is correct. If $RECIPIENT is not set on your server then you can either check main.cf to make sure that the second parameter of the maildrop script returns the recipient or you use the $HOST and $USER variables to built the recipient like this:
-A 'From: $USER@$HOST' -s 'Auto Response: from $USER@$HOST' \
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