To send a message to all the people currently subscribed to the list, just send mail to firstname.lastname@example.org. This is called "sending mail to the list," because you send mail to a single address and LISTSERV makes copies for all the people who have subscribed. This address (email@example.com) is also called the "list address." You must never try to send any command to that address, as it would be distributed to all the people who have subscribed. All commands must be sent to the "LISTSERV address," firstname.lastname@example.org. It is very important to understand the difference between the two, but fortunately it is not complicated.
The LISTSERV address is like a FAX number that connects you to a machine, whereas the list address is like a normal voice line connecting you to a person. If you make a mistake and dial the FAX number when you wanted to talk to someone on the phone, you will quickly realize that you used the wrong number and call again. No harm will have been done. If on the other hand you accidentally make your FAX call someone's voice line, the person receiving the call will be inconvenienced, especially if your FAX then re-dials every 5 minutes. The fact that most people will eventually connect the FAX machine to the voice line to allow the FAX to go through and make the calls stop does not mean that you should continue to send FAXes to the voice number. People would just get mad at you. It works pretty much the same way with mailing lists, with the difference that you are calling hundreds or thousands of people at the same time, and consequently you can expect a lot of people to get upset if you consistently send commands to the list address.