The Outpost firewall from Agnitum has an Attachments Filter plug-in that monitors e-mail as it is downloaded and allows you to delete or rename attachments on the fly. It’s based on based on file extension, which means it doesn’t handle embedded scripts; therefore, it won’t prevent a script from downloading and running an executable. I don’t recall seeing any stand-alone utilities that do this, but I wouldn’t have paid much attention if I had since Outpost does this for me.