The 404 error just means that the file with that name is not in the root of the site. If it were there you would not get this error. So we need to figure out where the file is on the live server in order to determine why you are getting this message. In order to figure out where the file is on the live server I will need more details about the structure of the live site and where you are uploading the pages to. Some screen shots from your ftp would be the most helpful. I suspect that you have this file in a sub folder on your live server and this is why it is not found with this URL. If you can post back with the details about your site structure and where this page is within that structure I will take a look to see if I can help you get this worked out.
If you take the TestMail.php page and add it into the root of your site the link to this page should work.