anti-spam for exchange

We are currently running GFI ME on our front-end server to catch spam but are not to pleased with the results of the product (too much false positives and false negatives, not possible to create personnal black/white lists, ...)
Exchange IMF looks like the easiest solution to implement but is not supported on a cluster.
So we are looking for somehting that is
- working with clustered exchange 2003 servers
- multi language sensitive (not all Russian e-mails are spam)
- learing by uses input
- allow users to create their own whitelists (for e.g. newsletters)
- allow users to check the spam they received
- return a custom NDR for all possitive messages

I thought about Barracuda but that is not quite cheap.
Any suggestions?