Domains protect their email addresses using Sender Policy Framework (SPF).  Yahoo Groups doesn't use the SPF worldwide standard, they have their own Domain Keys verification system.

Consequently, when a AOL customer is a member of a Yahoo Group, and the email goes out, Yahoo (in their infinite wisdom) will send out the email using the AOL customer's email address... which violates the AOL trademark and AOL's SPF Policy.  Recipients of such messages are instructed by AOL to REJECT such messages that do not originate from an authorized AOL mail server.

To get around this problem, tell the 1USA Webmasters that you are a member of a  Yahoo Group, and an over-ride can be put in place so that you will receive the emails from that Yahoo Group.

For more info about SPF, visit http://www.1usa.com/spf.html