i noticed this today. i started receiving bounced mails from yahoo! groups. it appears that spammers are now trying to spam the yahoo! mailing lists. cute. they would invent an e-mail address and add that to my domain name and then use the result e-mail address to try and join a group, send mail to another group, etc.
nice try. but no dice. fortunately those guys at y! groups keep trying to improve the service.
it also noticed something in two or three of the groups i manage. an e-mail address would either subscribe via web or mail and after a few days it would unsubscribe. i would assume that the person was trying to mine the group for e-mail addresses or send out spam to the group. in all the groups i manage, database is only viewable by the owner or moderator of the group. they are also the only ones that can send to the group. no one else.