[Exim] exim 4 and NUL character

Hello

I'm using exim 4.14 with cyrus 2.2 and see several hundreds rejections a day
form cyrus with note "Message contains NUL characters".
After some investigation I found that these messages were originated by some
weak web-mail system but in any case I'd like to get rid of this rejection.

As a workaround I put 'transport_filter = /usr/bin/tr -d \\000' in lmtp
transport and on my tests it works great. But in the exim log these rejects
still happen.
After looking into cyrus distro I found nothing special about "Message contains
NUL characters"

My part of configure:
# Cyrus
local_delivery:
driver = lmtp
command = "/usr/local/cyrus/bin/deliver -l"
batch_max = 100
user = cyrus
timeout = 5m
transport_filter = /usr/bin/tr -d \\000

Any ideas why it happens ?

--
Dimitry