How to send emails from a site created on Wordpress?

Question

PHP

Hello!
There is a site on WordPress.
URL: stud.example.edu.ru , where example.edu.ru is a separate site
Operating system: Ubuntu 14
Problem:
An email is sent to clients right in the php code of the site. I'm using the standard php mail() function;
The letter is sent all right. But if you try to send an email to [email protected],
nothing happens.
Here is a simplified version of the code:

$email_addresses = ['[email protected]' , '[email protected]','[email protected]' , '[email protected]' ] ;
foreach($email_addresses as $addres){
   mail($addres,'Congratulation' , 'blah-blah-blah');
}

Then all recipients will receive a letter, except for the ill-fated '[email protected]' .
That is, letters are sent to all addresses, except for those that are related to my domain.
What should I do: somehow raise or configure the mail server, or maybe I have some
minor errors.
Thanks