Because they do not just have one server, they have a cluster of servers and also in different countries. This issue only effects yahoo servers and it is random because you may not necessarilly be being dealt with by the same cluster when you logon to server. They only need a few set incorrectly to effect people on a random basis. They are not interested in fixing anything because it does not effect their webmail which they would prefer you to use anyway. If it was a Thunderbird problem then everyone would have the problem who uses that server and this is not the case. Once in the past it effected me for a short period. I accessed webmail account, emptied junk and deleted a load of old mail that I had copies of anyway and emptied the Trash. I only kept the more recent last month worth of emails. It was soon up and running again and i've not had it since. I often wondered whether the yahoo server triggers this when the Pop account has a certain amount of old messages still on the server and they would like them removed to save space. So check to see how many old emails are still on the server.