Okay, I solved my own problem. I'm not sure where exactly the error was, but I fixed it by stopping Postfix, removing all lines in the #User configurable parameters
section of main.cf to start from scratch, then copy/pasting (making the required hostname changes) all of the postconf -e commands from the howto all over again, and starting Postfix back up. No more bounced e-mails and my e-mail headers indicate that amavis is busy scanning for viruses and spam.
Color me happy!