Your config is incorrect. Here're the correct settings:

$CONFIG->emailer_transport = 'smtp';

// $CONFIG->emailer_sendmail_settings = '';

$CONFIG->emailer_smtp_settings = array(
    'name'              => 'localhost.localdomain',
    'host'              => 'ni-chrome.guzelhosting.com',
    //'port'              => 25,
    'connection_class'  => 'login',
    'connection_config' => [
        'username' => 'user',
        'password' => 'pass',
        'ssl'      => 'tls',
        'port'     => '587',
         //'use_complete_quit' => '',
    ],
);

I was wrong in the previuos post. Here's the corrected version:

$CONFIG->emailer_transport = 'smtp';

// $CONFIG->emailer_sendmail_settings = '';

$CONFIG->emailer_smtp_settings = array(
    'name'              => 'localhost.localdomain',
    'host'              => 'ni-chrome.guzelhosting.com',
    'port'              => 25,
    'connection_class'  => 'login',
    'connection_config' => [
        'username' => 'user',
        'password' => 'pass',
        'ssl'      => 'tls',
        'port'     => '587',
         //'use_complete_quit' => '',
    ],
);

You shouldn't comment out the default port (25)

Thanks Nikolai,

Your sample settings above made me clear. I reedited the settings. And also I realised that the email address in settings in administration page (Site email address (used when sending system emails))  is different than I used in settings.php to set the smtp.

Now it works properly. Thanks for your helps. And I'm sorry for my carelessness which made here a bit busy.

Just Google it.

Try to delete log files from /your_data_directory/logs/html/

Thank you Nikolai. Deleting log files solved the problem.

If this is about Site notifications, have a look at https://github.com/ColdTrick/site_notifications_tools.

This allows you to set a retention period for the notifications, after the retention period the notifications will be removed.

Thanks Jerome! :)