[Exim] Senders Problem?

I'm having a problem with this director:

bounce_spam_master:
driver = smartuser
require_files = /etc/exim/bouncelist
senders = /etc/exim/bouncelist
new_address = :fail: "unknown user \"$local_part\" in domain
\"$domain\""

It generally works - but I did notice some messages sneaking through
that it should have filtered. I have run a few tests and if I set myself
up with a return address of the ones that get through - it does filter
me. But - I did notice something interesting.

My from header is:

From: user@???

but the message that gets through has from in all caps:

FROM: user@???

is it possible that the "senders" command doesn't work right id "from;"
is uppercase?