Basically, email subscribers send a message to the server with a specific address for the host server of the list of interest to the user. A word of warning is in order here.
Many lists have hundreds and in some cases thousands of subscribers.
Thus, these lists will generate literally in some cases hundreds of messages in a very short order. Many lists are moderated, meaning that there is a moderator who reviews messages prior to their posting. The most common problem encountered during attempts to subscribe is the use of incorrect format and specifically, lists that require your name instead of your email address and vice versa. After attempts to subscribe you will get an email back from the listserver that gives a result status that usually reads successful or unsuccessful depending on how you did. The following is a list of mailing lists related to emergency medicine and will hopefully help you start using mailing lists.