Can't send to other BT addresses

I was migrated to the new Yahoo webmail this week. Since then all my email works fine except that any messages I send to other BTinternet addresses doesn't arrive. Other destinations go without a hitch. Anyone else seen this?

Are you certain they aren't arriving? Could they be being treated as spam by BT and going into the spam bin? Seen this with gmail accounts if the sender "sends as" the gmail account from a non-gmail server.