I've the same problem which I have entered on other threads. I can't even change my password.
Looking at the number of threads regarding this BT have aproblem they really need to sort out.
HELP, HELP, HELP, HELP
At least by opening my mail through mail.yahoo.com I don't have the password problem. However I still have to do that before I get my emails into WLM.
Spoke with BT level 2 support 48 hours ago. He sounded competent and said it was a Yahoo/BT server problem which should have been resolved yesterday morning. He didn't phone back at 20.00 hrs yesterday as promised, nor has he tonight and the problem still persists.
I also received error msg 421 today when sending emails; when I googled this it suggested the servers were being worked on. However, having closed down the PC and opened it this evening, my unsent emails did go (after logging on via mail.yahoo.com).
Bottom line - problem still unresolved.
Touch wood, the problem seems resolved. BT never rang me back to check, despite their promise to do so after they said work was being done on the Yahoo server.
Been running OK now for a couple of weeks.