Gmail is bouncing

Over the last few days I've been noticing that not all my mail has been arriving—I'll see responses to messages I never got. Today I looked at my YahooGroups bounce history page, and 9 messages have bounced in the last week with the message "User over quota". But I'm only using 12% of my Gmail quota! Apparently others have been having this problem (and it's not just mail from Yahoo), but there's been no response from Google. This is really frustrating, since one of the main reasons I switched to Gmail for all my mail was because of YahooGroups bounces. Anyway, if you sent me mail recently and I didn't reply, please send it again (or post a comment here).

Edit: By the way, why do they make it so hard to find the "report a bug" link? I still can't find one for YahooGroups. (Their bounced messages list is listing message numbers that are greater than the highest message number in that group.)

Edit 2: I figured it out for Yahoo, you have to go to a specific help page and then click "No" under "Is this enough information?"