Shortly, a more comprehensive statement on viruses and hoaxes will be posted to the List. Bounced mail is list email that cannot be delivered to a listee. That mail is returned to the listmaster with a notice of undeliverability. As you can imagine, if mail bounces from only a few listees each day, I can suddenly be overwhelmed with 100 or more returning messages, on top of my regular email and posts from the List. A listmaster has no option but to delete the problem address.
Many lists do so automatically upon the first bounce.
At TWList, where I am on the verge of resorting to an automatic delete command, I let the bounces continue a short while, in case the problem clears itself. But then, I too have no choice but to delete the problem address. Should your TWList email stop arriving, please check that your mailbox is not full and that you have received non-list email. The latter will be an indication that your server is working okay. The message you receive in return will indicate what's what.