"anytime I receive an email attachment promising "something you'll enjoy" from someone I don't know.."

That's ok, but add this: "OR EVEN FROM MY MOTHER".

There are lots of viruses and worms and spies circulating around, which are sent by infected PCs without consent from the sender. A good trick to avoid becoming an unintentional distributor: enter !!!000 as an address in your list. If an undetected worm in your PC tries to send itself to your list, it will be stopped by Outlook Express rendering an error of "invalid e-mail address" and you will notice it. If you want to send a massive distribution to your list (hardly necessary), delete that address first.

Thanks for the tip on bounced mail: surely it is yet another trick to call your attention to the message.

Un abrazo - Andrés