BT, one of Yahoo's biggest customers for its Yahoo Mail service, has decided to dump the service in favour of its own BT Mail platform, which will be run by US messaging specialist Critical Path.
Yahoo Mail accounts have been given automatically to BT broadband customers for the past 10 years after it decided to abandon its own BTinternet email platform.
In addition to six million email accounts - most of them active - Yahoo will also lose a significant chunk of traffic to its portal.
Customers will keep their existing inboxes and folders, but will be prompted to update their passwords, according to BT.
The move follows a rise in complaints from customers, culminating in reports of a wave of BT Yahoo email account hijackings earlier this year, in which attackers were able to crack users' passwords and take control of their accounts.
The hijackings occurred after a cross-site scripting (XSS) flaw targeting Yahoo Mail was discovered in January, although Yahoo claimed that it had fixed the flaw that month.
Telecom NZ in New Zealand has also been affected by attacks on its Yahoo Xtra email service, resetting passwords on 75,000 out of 450,000 accounts. However, Telecom NZ said in April that it would be retaining the Yahoo email service.
However, it is not necessarily one-way traffic. In April, Sky Broadband migrated its email service from Google's Gmail to Yahoo Mail.
