Try searching your I.S P's website to see if you can log on to Web mail. You can delete it from there. I am with Virgin and can read my e mails when I'm away from home, and delete them afterwards. You will need your e mail address and password.
I think if the email is over a certain size you cannot shift it from your ISP webmail, I had this a few years ago when on dial up and somebody sent me a huge email attachment which completely clogged up my inbox on the server.
To clear the offending email goto click here which will give you an overview of your inbox and allow you to delete the unwanted email from there.
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