This is one of the most frequent errors with mailing lists. Listserv is smart enough to catch some administrative commands sent to a mailing list, other mailing list managers do not have this feature. Signoff is an often used synonym for unsubscribe. For privacy reasons, listserv offers the possibility that your subscription to a list will not be shown to people doing the review of the list. Use the command set listname conceal to hide your subscription.
Mail is the default.
Index means you want only an index of the mails, not the mails itself. Conceal means that your subscription will be hidden for the review command. Repro means that the sender of a mail will also get a copy via the mailing list. One of the problems with shorthdr is that e.g. the MIME header fields are removed. In that case you can set listname fullhdr , so that all the headers will be kept in the mail.