When you join any Yahoo group it defaults to e-mailing every message posted to the group unless you specifically tell it otherwise in the "Message Delivery" options. Please do not respond to the group's messages that are e-mailed to you with "why did you send me that?" or similar inane comments. You're receiving them by e-mail because you did not ask not to receive them. If you don't like getting them by e-mail then choose a different option for "Message Delivery" in your settings for this group.