OK - I have a workable system now - many thanks to all.
The only thing that still bugs my clients is that they don't want to see any spam.
I have looked and can't find a global filter solution for the Exchange box. (that doesn't cost $$$)
So - I'm still looking for a sme solution.
A friend of mine uses maildrop to filter the spam before passing it to his exchange - but he says it's overkill.
Has anyone set up maildrop on an sme system - and if so, how hard is it ??