[exim] Return-path rewriting or removal at transport level

I'm facing some problem with emails sent from my system
(debian etch + exim 4.63). It happens sometimes they don't
get received by their recipient, so I think they're filtered
probably because they have a Return-path header value
different from the From header value, or because their
Return-path value, corresponding to a local address (of the
form user@???) is unreachable.
Emails are filtered only by some providers and mailing
lists, included this.

I'm trying to remove the Return-path from all the *outgoing*
mails. The following snippet configures my smarthost transport:

remote_smtp_smarthost:
debug_print = "T: remote_smtp_smarthost for
$local_part@$domain"
driver = smtp
hosts_try_auth = ${if exists
{CONFDIR/passwd.client}{DCsmarthost}{}}
tls_tempfail_tryclear = false

# this rule replaces the From value in the headers if it
corresponds
# to that of a local user
# I can't rewrite the Return-path from here
headers_rewrite = *@+local_domains \
"${lookup{${local_part}}lsearch{/etc/email-addresses}
{$value}fail}" f

headers_remove = Return-path

The problem is that it doesn't work. The Return-path of all
the outcoming mails corresponds to the local address of the
user who sent the email.

What's wrong with it (sorry if this question implyes a deep
misunderstanding of exim configuration)?

Another possibility would be to replace the existing
Return-path with the value of the from header, but it seems
this isn't possible at transport time.

Any help will be highly appreciated.

Cheers