[exim] Error handling with transport_filter

Hi all,

I'm currently experiencing problems with exim if a transport_filter
fails. Exim (version 4.50) will defer that message (good!) but sends
an empty message (no subject, only minimal headers, no content) to the
recipient (bad).

Here is my transport:
> dspam_transport:
> driver = pipe
> command = /usr/sbin/exim -oi -oMr dspam-scanned -f '$sender_address' '$local_part@$domain'
> transport_filter = <command>
> current_directory = ...
> log_output = true
> temp_errors = *
> message_prefix = ""
> message_suffix = ""
> user = exim
> group = exim
> return_path_add = false

The transport_filter command may fail if the database is not available
(exit code is != 0 in this case). Defering the messages is good but
how can I suppress the sending of empty mails by exim?

--
Felix