You write a message using your mail reader and address it to the list: listname@sheperd.com. Then, you send the message.

Your mail provider passes the message along to sheperd.com

The list server checks to make sure you are authorized to send messages to the list. If not, the message is sent to the system administrator.

If you are authorized, the list server re-sends your message one-at-a-time to every address on the list by connecting with each mail server and transferring the message. Everyone with an email address on the list receives a copy of your message.

Mail servers reject messages if addresses are invalid. Rejected messages are returned to the list server. Depending upon the configuration of the list, these messages are either held in a local mailbox for the clerk, or forwarded to a specified address. The clerk must log in at http://sheperd.com:8383 and remove these invalid addresses. If you wish to change the destination for returned messages, contact the system administrator.

If invalid addresses are not 
removed, Here's the result:

Each time a message is sent to the list, "bounce" messages are returned to the clerk for undeliverable addresses. It's important to remove invalid addresses from the list.
          Bounced Messages