[exim] Outgoing mail : how to remove tags/keywords from the …

Hello,

we have a spam/virus filtering system that add spam score and other info
with keywords at the beginning of the subject header
to inform the end-user and help him write sort-rules in his mail-client.
Example : {spam: 43} {newsletter} {SPF: pass} {DKIM: No signature}, etc...

But when an end-user answers to that mail we would *remove* all these
tags (they are always between { } characters, so a regex search/replace
must be possible I hope).
The goal is that :
- the TO /CC /BCC addresses does not see these spam / info tags
- in case of repeated send / answer dialogues the subject line does not
fill up with these tags !

I did search and did find how to do this for *incoming* subject header
rewrite
(
typically sequences like :
headers_add "New-Subject: {spam}: $h_subject:"
headers_remove subject
headers_add "Subject: $h_new-subject:"
headers_remove new-subject
}

but I could *not* find something to do this for *outgoing* mail (as well
to local domains addresses or external domain addresses, all answers)
I saw... outlook exchange does propose this functionality, exactly for
the same purpose.
But we want to use Exim he :-)

Does someone know a FAQ where is explained how to do that,
or have experience with this?
or an example of instructions and where to put this in the exim.conf ?

Very thanks for your help !
Yves.