My Internet provider has given another customer access to my email account

ComputerUser

Please tell me what I can expect from my Internet Provider as I have discovered that my email has been duplicated by them and given to another User. That user could see all my folders and sent emails going back years! This is a serious security breach and all my sent private and financial correspondence has been made available to someone else. If you consider the amount of sent emails over years when a single email may not contain alot of information but when linked with other emails could be used to build up a picture and steal an identity.

I have contacted the provider they are looking into it but any advice would be greatly appreciated. What are my rights?:(