how to fix "500 - Internal server error." while sending newsletter in PHP -

I am trying to create a newsletter in PHP. I have a database that has more than 3,000 email addresses.

When I execute the code on the server, it shows a server error:

  500 - Internal server error is a problem with the resource you are searching for, And it can not be displayed.  

Here's my code:

Please help, thanks in advance.