Setting Thunderbird as a default mail client for the operating system will not in any way affect how it gets mails, or how the server responds to it. Having said that, if you have malware on your computer, it could be using the change to spam your inbox. But that is not something I can help you with at all. That is a task for an anti virus company. It my be that Talk Talk have options on their web site to limit the mail that is downloaded by a mail client, but as a general rule a mail client will download whatever mail is in your web mail account. On occasions anti virus email scanners die a horrible death and their method of demonstrating that is to repeatedly download the same email over and over. This used to be a particular party trick of Norton's. Generally disabling email scanning until the backlog of mail is downloaded will clear the issue. On other occasions the issue will actually be a corrupt email on the server. So log into the web mail using a browser, empty the trash and spam folders and delete whatever email is downloading over and over and the next mail in the list after the repeated download. .