[exim] Header syntax check and 8-bit characters

Hello,

I frequently get spam mails with 8-bit characters in the Subject that
are not RFC 2047 encoded, not even a charset given in Content-Type,
but a "MIME-Version: 1.0" header.

Are 8-bit characters in mail headers acceptable at all? I did not find
a place where RFC 2822 allows them (which does not mean much). I know
Exim allows them, because it is 8-bit clean and proud of it.

Are there legitimate MUAs generating such (today) broken mails? After all,
this is not Usenet, where such behaviour is unfortunately common.

I would like to come up with some condition under which rejecting those
spam mails would not violate existing RFCs and not reject legitimate mail.
Checking for header syntax already keeps much crap outside my mailers
and I would like to get even less of it.

Michael