I partialy agree with dspeidel.. there will be 2 markets the first being the Server Side filters.. the second the client side will endup being part of the AV package (we are seeing it now). It would be better, as dspeidel commented, as part of the Email Client software but the AV Co's will convince consumers that their product is the only way to go.. geez integrate Mailwasher with any of the clients and your on a winner..

the end barrier.. if the Anti-spam tool is part of the client software.. it is another setting the user has to fiddle with when installing their machine, or upgrading their software.. it will be missed..because it involves more settings and is to much to think about
when it is part of the AV prog.. people will run with the default settings or just disable the tool... because it involves more settings and is to much to think about .. and they will assume that the AV will take care of everything...