[Exim] Filtering messages by recipient count

Hi, all!

I need to filter messages, sent by one of users. His messages must have
less recipients (in To:, CC, :BCC fielsa), than some constant value. And
if recipient count is more than this constant, message must be returned
to sender with corresponding note.

I can write an external script, that validates messages, but i cat't plug
it to exim correclty.

If i write (user filter)

    # Exim filter

    pipe "path-to-script"

then valid messages not delivers, and if i write

    # Exim filter

    unseen pipe "path-to-script"

then invalid messages does not returns to sender.

exim 3.22