[Exim] inconsistency with non-ascii

I have a problem with handling header fields containing non-ascii text. (We seem to
be getting more and more such messages!)

This is exim 3.33

I have an autoreplyt ransport containing:

subject = Failed: ${if def:h_Subject: {$h_Subject:}{alternative text}}

The message log for messages with non-ascii text in the Subject: field contains the
failure: "Expansion of "Failed: ...." in the xxxx transport contains non-printing
character 177.

I would have thought that exim should either refuse to accept messages at the SMTP
stage if they contail illegal headers, or it should accept non-ascii in the
string-expansion. Message like this which do not hit this particular transport go
through for delivery.

Do I have any way round this?

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