The 500 Internal Server Error is generic, the real cause is hidden in your server logs. I cannot check the error log unless I get access to cpanel. Can you pls check once the error reported in the error log link posted on cpanel? Does not seem to be a permission issue but we need to see the exact error being thrown when we make a request.
None of the php files are accessible from the browser client on your server, not just sendmail.php. (another example is css/skins/skin.php)