[Exim] Catching bounce messages

Dear All,

I am trying to intercept bounce messages passing through our exim box.
The reason for this is that we want to limit their size (some braindead
mailservers in this company send out huge multi-megabyte bounces where
just a plain text with some headers would suffice).

I have created a router (dnslookup_chop) and a transport
(remote_smtp_chop) and placed the router before the existing dnslookup
router. It looks like this:

dnslookup_chop:
driver = dnslookup
condition = ${if eq{$header_from:}{<>} {1}{0}}
domains = ! +local_domains : ! +relay_to_domains
transport = remote_smtp_chop
ignore_target_hosts = 0.0.0.0 : 127.0.0.0/8
no_more


The remote_smtp_chop just uses a filter (shell script) to reduce the size
of the email:

remote_smtp_chop:
driver = smtp
transport_filter = /usr/exim/filter/chopchop


However - in testing (sending email with sender address <>) it works
fine, but on actual bounces it never gets triggered. Even though in the
exim log I see the bounces coming in like this:
2003-06-08 09:23:45 19OuWn-00049K-5b <= <> H=xxx.yyy.com ...

the from header does not actually contain <> . Who knows how to catch
bounce message and can think of a good condition?

Cheers,Rutger