[Exim] pipe outgoing messages

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Greetings,

I'm trying to set up Exim to pipe outgoing messages through tmda's
"tmda-sendmail" when it sees a custom X header. This would enable
pretty much any MUA capable of specifying custom X headers usable with
tmda's outgoing client features. Ideally this would be done through
the individual user's Exim filter file (so each X header could be
unique to the user), but am I correct that outgoing mail does not
consult this .forward file? When I set up this pipe as a system filter
entry it calls the pipe okay but it fails with an environment error.

Here's the system_filter entry:

if $message_headers contains "X-insert-tag-here" then
    pipe "/usr/local/bin/python /usr/local/tmda/bin/tmda-sendmail"
seen finish
endif

And the error output by Exim is:

      Child process of address_pipe transport returned 127 from
command:
     /usr/local/bin/python

------ pipe to |/usr/local/bin/python /usr/local/tmda/bin/tmda-sendmail
        generated by message filter ------

env: python: No such file or directory


I kind of thought specifying the full path to python would take care of
any path issues. Incoming messages get piped through a tmda-filter
just fine with a similar pipe in the user's Exim filter .forward file,
so I'm guessing that there's something with running it as a system
filter.

I was hoping someone might be able to clue me in to the proper way to
do this. Or am I going about this all wrong?

Thanks in advance,
Todd

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