[exim] How to Unfold Subject Line ?

I'd like to have Exim unfold Subject lines that wrap across multiple
lines.

Anybody have a recipe for that to share ?

At first I thought a system filter would work

# Exim filter

if first_delivery
then
    headers add "Old-Subject: $h_subject:"
    headers remove "Subject"
    headers add "Subject: $h_old-subject:"
    headers remove "Old-Subject"
endif

But the "\n" symbol is carried along too.

Second thought was rewriting headers at transport time, but the
Subject line does not have it's own identifier. I can unfold all
headers, but I don't want to do that.


I haven't come up with a third idea yet.

Surely I'm not the first person to want to do this, but I haven't
found anything in the Exim archives.

Got any clues for me ?

Thanks

mark