Thanks for your answer Jeremy. Our clients are still running on Ex 2003 so probably this is why I am not fully understanding you.
My confusion is a bit related to the fact that Exchange is available in two editions Enterprise and Standard.
But you can then also have a mix of Enterprise CALs and Standard CALs.
Let's consider some scenarios:
1. Exchange 2010 Standard Edition with a mix of Standard CALs and Enterprise CALs
2. Exchange 2010 Enterprise Edition with a mix of Standard CALs and Enterprise CALs
The question is what type of anti-spam filtering will Standard CAL users get?
Will there be 2 different spam filters one handling emails addressed to Standard CAL recipients and another (forefront) to Enterprise CAL recipients?