I had the same problem with an email provider too, they didnt solve it, and it still happens now, so i cant help there never having my problem solved for me...
Sure BT will help, they have been great to me, and i currently dont have that problem with BT. But i dont know if someone signs in and out at the same time as me using the same account details...
Its a strange one and wouldnt mind knowing the answer, maybe it will work with my other email system??