Response.Redirect("mailto:");

Hi,
I have a button on my page which grabs the text from a TextBox and creates a mailto link:
Response.Redirect(string.Format(mailto:{0}, txtEmail.Text));
I tried this on one page and it works fine; it opens the email client, and when the email is sent or closed I am still on the same page.  However, on another page it opens the Email client, but when the email is sent or closed, the URL of the page has changed to mailto: <email address>.  This didn't happen with the first page, it just stayed open.
Does anyone know why this functionality might work diffrerently on separate pages?  Alternatively, does anyone know a better method of opening the email client when the button is clicked?
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