[exim] spam forwarding

We run exim 4, spamassassin and sophos on our mailhub.
Our policy is to mark incoming spam in the subject line with the word spam
and deliver the message. The end user can then filter messages out.
I have now come across a problem that a message marked as spam was sent to
an exim mailing list; the list has a member with an email address at an
external site which has a policy to junk all spam, and is seeing us as
sending them spam. The same could happen with a .forward file to another
site.

What is best practice is such situations?
how is this best resolved?

Many Thanks
Judy Angel
University of Hertfordshire