window.location() not working, not opening page

Try using replace function instead

window.location.replace('http://www.rainbowcode.net/index.php/profiles/mail?='+mailid)

I spent 2 days trying every solution shown here and elsewhere, to no avail. Then I removed the form tags, which served no purpose since there was no submit button, and the problem went away using:

window.location = 'mypage.php', true;

If you need to open a new window, you should use the window.open() method. window.location refers to the current windows address, and will only - when using window.location.reload() - reload the CURRENT window.

Setting the location works just fine, but then the form is submitted, which will reload the current page instead.

Return false from the method:

function sendmail() {   
  window.location.href = "http://www.rainbowcode.net/index.php/profiles/mail?="+mailid;
  return false;
}

and return that status in the event to stop the submit:

<input type="submit" value="Send" onclick="return sendmail()">