Copied From Yahoo:
If group messages sent to your email address are returned to Yahoo! Groups as undeliverable ("bouncing"), your Yahoo! Groups account may be temporarily disabled.
Messages can be returned for several reasons:
* Your mailbox may be full -- try deleting messages from your Inbox.
* Your ISP may have a backlog of email to process -- contact your ISP.
* Your ISP may be blocking email from Yahoo! Groups -- contact your ISP.
In many cases, Yahoo! Groups can automatically reactivate your account once the delivery problem is solved. However, in some cases you will need to manually reactivate your account:
1. Visit your My Groups page.
2. Look for a bounce alert near the top of the page.
3. Click the Alert link to reactivate your account.
Once an account is considered bouncing, Yahoo! Groups will send a series of test messages to the member. If the test messages are returned as undeliverable, then the account will continue to bounce. However, if a message is delivered and not returned as undeliverable within five days after it was sent, the account will automatically be reactivated*. Members may also manually reactivate their accounts by visiting their My Groups page."
*This may or may not be true.