[exim] changing envelope from within an user filter

Hi,

I'd like to run a filter per recipient (however filters are managed by
the admin), basically something like

filter:
driver = redirect
domains = ${if exists{CONFDIR/virtual.d/}{dsearch;CONFDIR/virtual.d}{}}
file = CONFDIR/virtual.d/$domain/$local_part
user = Debian-exim
allow_filter = true
allow_fail = true
allow_defer = true
reply_transport = filterreply
directory_transport = filterdirectory


However, I'd like to be able to change the envelope from address when
forwarding to others. 'errors_to' seems however only to be supported
in system filters. However, in a system filter, I cannot adjust for
just one recipient but have to take care of the full message (if I
understand that correct), especially it is only run once even if
multiple of my recipients would qualify.


What would be the most appropriate way to set the envelope from
address in this situation (apart from replacing my system filter with
a hand-crafted router)?


Regards,
Andi

PS: This is my second try in sending out that mail (with a bit
different subject), the first one didn't reach me back (and I can see
it neither on the list page), so I hope noone is getting this as
duplicate message.