Sorry to drag this one out, but we have tried with some test clients, and everything looks/works fine.
Obviously though, we may not notice what we can't see.
We have tried opening the same mail databases with both 6.0.x & 6.5.x clients, and the interface seems to appear the same.
If the 6.0.3 users don't notice anything missing, they probably won't complain or miss it, and it will then become additional functionality once they are upgraded to 6.5.x.
However, if there is code which may prevent the correct operation of the messaging features for these clients, then that is something we have to consider more closely.
Thanks again in advance,
T.