If you're using Courier-MTA to do the final delivery, then each user has the power to make extended aliases work or not using dot-courier files in their home directory. For instance, if the "linux" user wanted "linux-kde" to be a valid address, he would create a file called ".courier-kde" in his home directory. That file should contain instructions that indicate how the mail should be delivered to a mailbox or alternate address. This is documented in the first section of the dot-courier man page.