[Exim] bsmtp and suffixes options

I am trying to pass email addressed to local users (optional with a
suffix) to a bsmtp transport. I set up a director:

spam_check_director:
driver = localuser
transport = spam_check_transport
suffix = -*
suffix_optional

and a transport:

spam_check_transport:
driver = pipe
bsmtp = one
pipe = /some/command -u ${local_part}

The problem is that an address such as pdw-foo@??? seems to
get rewritten to pdw@??? when passed to the bsmtp pipe.
The reason I use the localuser driver with an optional suffix is because
I want to pass the local username matched to the pipe command, but I
want the envelope-to in the bsmtp pipe to be the original envelope-to
(suffix and all).

Is there anyway to achieve this?

Paul