[Exim] More on exiscan demime facility

We are running exim 4.12, exiscan 4.12-25 and clamd 0.54.
We have experienced that when we have exiscan_demime_action = pass some viruses get through.
Now we have tried to use exiscan_demime_action = redirect to see what kind of messages that get rejected by this and we have found several that seem ok.
The usual reason for exiscan are:
X-Broken-MIME: base64 line length exceeds 76 characters
X-Broken-MIME: double Content-Type: header

These mails are mainly coming from yahoo.com and spray.se.
We have tried to use yahoo and send mail with a usual word document as attachment and get the same result.
What can be the reason for this?
We are a university and a lot of students use yahoo and spray to send theire work to theire teachers which means regarding to this that we can not use exiscan_demime_action = reject
Is there any solution for this?
Grateful for help.

Regards
Nicklas Hardman
University of Borås
Sweden