We have been contacted by several people over the last few days about problems with BT email accounts (particularly those at the btinternet.com domain).
After a bit of searching, we discovered the following message on the BT web site.
The problem appears to be specific to those email addresses that use the BT Yahoo! email system (as opposed to those that use Microsoft Office 365).
If you are affected by this problem, it looks as if you will probably need to change your password.
The instructions for changing your password are available on BT's web site: www.bt.com/secure