[Exim] Help please

Hello!

    My external programm gets 2 parameters: $local_part and $message_size.
It returns 0 on success and writes forwarding address to stdout and 1 on
fail and writes nothing to stdout. What should I write in filter to make
it work properly?
1. If my_prog returns 0 (success) deliver to address it printed to stdout
2. Else there should be no delivery

Can't understand the logic of string expanding and testing...

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Good Luck,
      Boris [Team OS/2]