Just looking at the source to a spam message and I noticed that Yahoo Groups is now adding a set of headers to help track the origin of spam:

 X-AntiAbuse: This header was added to track abuse, please include it with any abuse report
X-AntiAbuse: Primary Hostname - cpanel2.nocspeed.com
X-AntiAbuse: Original Domain - yahoogroups.com
X-AntiAbuse: Originator/Caller UID/GID - [32051 552] / [47 12]
X-AntiAbuse: Sender Address Domain - cpanel2.nocspeed.com

This particular message isn’t a spam message, but very interesting data here nonetheless. Check it out, and if you do get any spam through a Yahoo Groups list, include it in your spam report and let’s see if they act on it too.