I use Outlook Express as my email program, how do I prevent emails also appearing in Yahoo mail inbox?

I use outlook express as my default email program, this is password protected, i.e. I have to deliberately initiate a download of email into the inbox. My problem is that the same emails are already available for view in my BTyahoo mail inbox without any intervention from me. How can I prevent this happening and restore the security of my emails?
I use Outlook Express as my email program, how do I prevent emails also appearing in Yahoo mail inbox?
Yahoo must be your ISP and, as your Internet Service Provider, all the emails coming to your assigned address will first arrive in your yahoo email server. They will sit there until you open up your Outlook Express email. I have the same set up. I like to go to yahoo at the very beginnig and set up all my email and spam blockers at this point so that very few spammers get thru to my OUtlook Express programme.