This is particularly troublesome over the summer break and winter holidays. Anyone who fails to reply gets unsubscribed. When a user is unsubscribed in this fashion, the listserver sends a final message with instructions for getting back on the list. I think that some of these lists are smart enough to not send requests for confirmation to members who have posted to the list recently. I think that this is an excellent feature because many folks who subscribe to lists never figure out how to get off them or never bother.
Some folks get a new account and never bother to deal with mail still coming to the old one.
I encourage anyone who runs a high volume mailing list to consider using a deadman switch to ensure that mail is going to mailboxes that are actually being read. It would also help if mailing list administrators would regularly send out a brief message with information on how to unsubscribe. From the simple Caesar cipher to RSA and beyond, there is nothing the book doesn't at least touch on. Protocols, techniques, algorithms, and even source code are included. For those who do need more rigorous treatments of specific topics, the bibliography lists almost a thousand references, all of which are described and cited within the book text at some point.