You can only post a message from the email address that you subscribed to the list with. If you send a message from an address not registered with the list it will automatically be bounced.
Login to the members' section, and then on the "Member's account" page - follow the "change email" link which is next to your displayed email address.
To unsubscribe or manage your discussion list subscription go to http://lists.pcst.co/mailman/listinfo/pcst-members, then:
.Uder the heading ‘PCST-Members Subscribers’, you have options to unsubscribe, or get a password reminder, or change your membership option.