Overrides Drupal\Core\Mail\PhpMail::mail().
Accepts an e-mail message and store it in a variable.
Overrides PhpMail::mail
public function mail(array $message) {
$captured_emails = state()
->get('system.test_email_collector') ?: array();
$captured_emails[] = $message;
state()
->set('system.test_email_collector', $captured_emails);
return TRUE;
}