Follow these steps to help diagnose why a user may not be receiving Yahoo! Groups messages.
# Go to the group website, click on Management > Manage Members, > Bouncing, see if the user is listed. If they are listed as Soft Bouncing, follow the instructions to try to reactivate them. If they are listed as Hard Bouncing, ask the user to resubscribe.
# Go to the group website, click on Members, Members, search for the user, make sure they are set to receive Individual Emails or Daily Digest (not No Email).
# Ask the user if they are receiving other Yahoo! Groups email at their same address. If yes, and if you have administrative access to those other groups, check that the user is in fact ubscribed at the same address. If yes, their mail client software is likely hiding the specific group’s messages from them for some reason.