It has been going on for years and BT are hopefully sorting it out. They are moving away from Yahoo during the course of this year to a new email system so hopefully the problem will go away then.
And if that fails, perhaps BBC's Watchdog programme might be interested ...
... or not, given that it's a Yahoo! problem and BT are already dealing with it, as already explained.
The advice from the Ofcom website is:
To get help with your individual problems, follow these steps:
Contact your provider’s customer services department and explain your problem.
If this doesn’t resolve the issue, make a formal complaint to the company. You should find details of how to do this on the back of your bill, on their website or ask their customer services.
If your provider is unable to resolve your complaint, ask for a deadlock letter. This enables you to take your complaint to an Alternative Dispute Resolution (ADR) scheme.
ADRs act as an independent middleman and will examine the case from both sides and reach a decision they think fair. If eight weeks have passed since you first formally complained you can contact the ADR directly.
If all else fails give that a go
This does not work for email problems because Ofcom do not regulate email provision. Because of this the appropriate ADR scheme that BT subscribe to do not handle it also because Ofcom do not. A government e-petition has been raised to get this ludicrous situation rectified.
At the moment the Daily Mail seem ahead on points and got someone £100 compensation (finance letters page today) might be worth a try.