[exim] "Vista Mail" and errors after DATA

Hi,

In our exim configuration we use the MIME ACL to refuse mail containing
attachments with specific extensions, which has been working happily for
some time. Unfortunately, we've now run across a problem with this technique
when "Vista Mail" (Windows Vista's replacement for Outlook Express) is being
used as the MUA for a local client.

Errors created during the RCPT ACL are correctly handled, with Vista Mail
returning the error message from exim to the client. However if the reject
occurs later in the process (in this specific case during the MIME ACL)
Vista Mail silently ignores the fact that a 550 was returned and simply
moves the outgoing mail in to the "Sent Items" folder with no indication to
the user that the mail has not been sent.

Any suggestions as to how to handle this would be greatly appreciated. I'd
really rather not exempt local clients from the relevant ACLs and telling my
boss he can't use Vista Mail is also sadly not an option.

Thanks,

Adam