Learn to Use your Computer

Computer accounts are created and deleted often enough and mail addresses change often enough that a large list always contains, at any given moment, some addresses that are no longer valid. If you send a message to the list, your message is forwarded to these invalid addresses, and a return message reporting the bad addresses is generated for each of them. Mailing-list managers (both human and computer) normally try to deflect the error messages so that they go to the list owner, who can do something about them, rather than to you. As often as not, however, a persistently dumb mail system sends one of these failure messages directly to you. Just ignore it because you can't do anything about it.
Sometimes you may get an "I'm away on vacation" message or a "click here if you're not a spammer" message in response to list messages you send. Don't respond to those either since vacation and antispam programs shouldn't be responding to list mail at all, but do forward them to the list manager so he or she can suspend their subscriptions until they get their software under control.