[exim] Spam filtering in external application

Hi

I want to use spam filtering on my exim 4.69 installation.
On various sites I found configs for a transport like this:

dspam_spamcheck:
driver = pipe
command = "/usr/sbin/exim -oMr spam-scanned -bS"
transport_filter = "/opt/dspam/bin/dspam --stdout
--deliver=innocent,spam --user $local_part@$domain"
use_bsmtp = true
home_directory = "/tmp"
current_directory = "/tmp"
user = nobody
group = mail
log_output = true
return_fail_output = true
return_path_add = false
message_prefix =
message_suffix =


With this config a message is given to the external application and the
output is reinjected into exim with a different protocol (spam-scanned in
this case).
In the router section this can be checked with {eq
{$received_protocol}{spam-scanned}}.

This method works, but I don't like that the message is run twice through
exim. I get two complete deliveries in the logfiles and I can't match them
because they have two different message ids. And I think the performance of
two complete message runs isn't good either.

I tried using the transport_filter directly without this reinjection:
E.g.

remote_smtp:
driver = smtp
transport_filter = "/usr/bin/dspam --stdout --deliver=innocent,spam
--user global --debug"

Looks like this method works too. Is there a reason why this method is
inferior to the one above?
Is this the "right way" to do this or is there a better one?

It is not possible to change the message body in a router and pass the
modified body on to the next router, right? (like unseen with modifying the
body)

Thank you for your help
Dave