Jason is way off on this one, and I don't think Planoie really explained what the problem is.
But first of all, I have to say that this project *FEELS* like an attempt at identity theft. Why would you want to act as an intermediary between a user and Yahoo??? Why would anybody TRUST you to act as an intermediary?? After all, you would be able to capture their usernames and passwords and all other personal info. You wouldn't catch me using your site.
If this *is* an identity theft attempt, then I'm happy to say I don't think you will succeed. But even if you have legit reasons for this, I don't think you will succeed. Here's why:
to set the value of some form field and then has a meta-refresh that submits the form. If it doesn't see the expected value in the form field, it knows that JS
didn't set the value, and so it gives you that message. I can think of a handful of other ways to make such a check.)
In short, I think this whole experiment is doomed to failure from the start. The fact that you had to ask the question in this forum tells me that you aren't somebody with the expertise to pull of a browser simulation and thus fool Yahoo.
If all you want to do is give your users a spot on your page where they can view their Yahoo mail, then there is an easy answer. Just use an <IFRAME> or <FRAME> and load Yahoo into it. (Of course, Yahoo might have some frame-busting protection built in, so that wouldn't work. Dunno.)