Here is a modest suggestion for email clients: when you forward a message, you should put some marker of this at the front of the Subject: header. Similarly, if you are configuring your email client and you have a choice, you should set it up this way.

(Some but not all clients do this today.)

Many anti-spam systems that I've seen put a spam status marker at the start of the Subject: line; it's a relatively obvious place, and putting it at the start increases the chances that the users will see it (instead of missing it at the end of the line, or having their client not even show a specialized header).

(Also, there is the whole tradition of putting 'Re:' at the start of the subject line for replies.)

Now suppose one of your users forwards a spam-tagged message. This forwarded message is not itself spam (the obvious example is 'dear system staff, why did this get classified as spam?'), but if your client leaves the start of the Subject: header alone, the status marker is completely intact. Filtering badness is quite likely to occur. Putting the forwarding marker at the start of the Subject: line avoids this (and plays along with the existing 'Re:' tradition).

(Having written this I have to admit that my mail client currently isn't set up this way. I should fix that, but there always seem to be more important things to do than fiddle with the guts of my MUA environment, which is somewhat baroque.)