[exim] non-ascii in the envelope

I am seeing an increasing number of messages which contain non-ascii characters in
the envelope sender. I am inclined to reject them.

The motive is that other sites seem to be doing so and I am getting frozen bounces
when user-forwarding tries to pass such a message on.

I see references in the list to people wanting to rewrite accented characters and so
on using the ${rfc2047:} expansion function so that headers with umlauts and so on
will be properly represented. However, it seems to me that this is the job of the
UA which first submits the message, not the MTA.

Can Exim be told to fail invalid addresses when 'verify = sender' is used in an ACL?
(Or some other way?)

If not, I am thinking of camparing $local_part with ${rfc2047:$local_part} and
rejecting if they do not match. Two questions:

1) Is there a good reason not to reject when non-ascii characters are in the
envelope sender?

2) Would my suggested comparison be likely to produce any false psitives?

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