The most difficult aspect of working with mailing lists is the distinction between the list itself and the program that controls the list. Both have email addresses, and both are usually on the same machine, but they have very different purposes. When you look at a mailing list index (there are a number of them on the www.) you will see something like this.
THE HIPPO LIST
The controlling program usually has a name like majordomo or listserv. You will probably only send mail to this address twice, once to join the list, and once to leave. Any mail sent to this address is NOT sent to a human, but to a program. The program can only handle very specific input, so you must type exactly what you are told, or the program will reject your request. If you say "Please subscribe me to the hippo list" the program will not understand and will reject you. Majordomo (or listserv) is a PROGRAM, not a PERSON.
The address you will use more often is hippo@nml.zoo.org. This address is the common address for the group. It is also sent to the majordomo program, but it is 'bounced' from there to the address of every other member of the group. Anything you send to the list address will be read by human members of the group. (and maybe hippos, but they are pretty tough on keyboards)