Guest Entries - How I can send message to user after saving form information I use Guest Entries plugin to add information, and I need to send some message to user who filled the form to email. Can you show me an example of how I can do this?
You could do this with a small plugin that listened to the Guest Entries afterSaveEntry event.

The logic would look something like this:

use craft\guestentries\controllers\SaveController;
use craft\guestentries\events\SaveEvent;
use yii\base\Event;

// ...

Event::on(SaveController::class, SaveController::EVENT_AFTER_SAVE_ENTRY, function(SaveEvent $e) {
    // Grab the entry
    $entry = $e-&gt;entry;

    // Assuming `toEmail` is a custom field on the entry.
    Craft::$app-&gt;getMailer()-&gt;compose()
        -&gt;setSubject('Thanks for your submission!')
        -&gt;setTextBody('You are amazing.')
        -&gt;setTo($entry-&gt;toEmail)
        -&gt;send();
});


You can generate Craft 3 boilerplate plugin code using https://pluginfactory.io/ and the official Craft 3 Plugin docs are here: https://docs.craftcms.com/v3/extend/plugin-guide.html