When your request to SUBSCRIBE is received by SPNAC, an email reply will be sent to your inbox, and you must reply to that email in order to be subscribed to the SPNAC mailing list. You don't have to write anything in your reply--just hit "reply" and "send"--but if you do not reply to that email, you will not be subscribed to the SPNAC mailing list.
If your request is to UNSUBSCRIBE, you will simply receive an email confirming that you have been removed from the mailing list.