[Exim] cyrus NUL failures

Since moving to the cyrus imap/pop server I am having a problem with cyrus's refusal
to take messages with Null characters.

If a user manages to send such a message it passes through exim OK but is rejected
by cyrus. exim generates a non-delivery message and tries to deliver that.
However, the non-delivery contains the Null character too so that is rejected and
the non-delivery gets frozen.

How do I intercept this and generate a non-delivery message which does not contain
the Null character?

I have been thinking along the lines of using a system filter which looks at the
non-delivery message body to detect the cyrus rejection message, but how would I get
it to truncate the message body before the Null character?

Any ideas would be welcome.

Phil.
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Phil Chambers (postmaster@???)
University of Exeter