Shandra wrote:

Ah!!!  I get it now, it was because of those 3 or 4 spam messages that went out in a row.  Our email systems blocked them, and in return, the listserv unsubscribed us…

It is unlikely that those few spam messages alone were responsible for Yahoo (and others) to start rejecting messages from this list. A more likely scenario is that someone sent out THOUSANDS of spam messages through the same email server this list uses (or one that it hops through). Yahoo then flagged that server as being a spam provider and started blocking it, sending back the automated messages Boris quoted. The listserv software didn't pay attention to why the messages were bouncing, just that they were for 5 days.

Unfortunately I don't have a better suggestion for avoiding it in the future than what Eleanor proposed.

-- Logan --