WordPress sends email using apache username

Description
Wordpress sends emails using the apache username, that is www-data@domain.com and I do not find any way to change the username of the email from www-data to admin. Any help is very appreciated!

Steps taken
1) I have installed ssmtp to take care of sending email and configured it properly (it sends email with the root name as freely specified in the config file).

2) I tried to add the following lines in the functions.php of my current theme:

// Function to change email address
function wpb_sender_email( $original_email_address ) {
    return 'admin@domain.com';
}
 
// Function to change sender name
function wpb_sender_name( $original_email_from ) {
    return 'Admin';
}
 
// Hooking up our functions to WordPress filters 
add_filter( 'wp_mail_from', 'wpb_sender_email' );
add_filter( 'wp_mail_from_name', 'wpb_sender_name' );

But I still receive emails from www-data@domain.com.

So I do not know where should I set the username and avoid using the apache username.