Drupal8 emails end up in spam. How to fight?

Question

Drupal

I send emails using the SwiftMailer module, but some users' emails end up in spam, for example, those who are registered on gmail.com.
Letters are sent like this:

$mailManager = \Drupal::service('plugin.manager.mail');
....
$result = $mailManager->mail($module, $key, $to, $langcode, $params, NULL, $send)

;
and are formed via hook_mail

Answer 1

Try sending via SMTP.
If already, check if spf, dkim, dmarc are configured correctly.

Answer 2

https://www.mail-tester.com/