There is a known problem with some mail list software which refuse to accept your email address when you request a singoff from the list. As it is their problem, the only way (and the recommended way when the automated method does not work) is to ask the list-owner to handle it for you. The appropriate address to send such a request is normally given in the welcome message, or a periodic information posting, from the list. If you don't have the info page, most servers support a request in the form of a message containing the line info listname in a message to the listserver command address at the site. That may be listserv, listproc, mailserv, or majordomo, (or sometimes something else), you can usually figure it out by looking at a message from the list. If that is not available, standard forms to reach the administrator are listname-request@site, or listname-owner@site, where you would substitute the actual listname and site for a message to or from the list. If that still does not work, mail to the postmaster at the same site should be able to identify the list-owner address. The postmaster does not usually run lists, he has too much else to do ;-)