Any chance you could paste lines 10-12 of /home/newslett/public_html/persianmail5.tk/back/newsletter.php
It may mean you need to put quotation marks around "title", "body" and "receipt" in those lines. But it may not. If you paste the code the problem will become clear, or someone may be able to suggest how you can debug it.
This error occurs when you try to access an array index with an invalid key. So, somewhere you're accessing an array with $variable['title'] where the key 'index' does not exist. Try dumping the entire array with var_dump($variable) to see a list of existing keys.