Problem with Mail client after sending to a Yahoo address?

I sent an email to a yahoo email address (as part of Yahoo Group moderator duties), and have received repeated "Delivery Status Notification (Failure) messages saying the yahoo address is not a good one. The problem is that after sending this email, my mail client (Apple Mail) no longer downloads email from my mail servers that sent to that address. I.e. I can see incoming emails via webmail but they no longer download to my mail client. What happened?