code: public function getEmailMessages($to = '')
      {
          // Update address for checking.
          if (!empty($to) && $this->email !== $to) {
              $this->email = $to;
          }
  
          if (empty($this->messages[$this->email])) {
              $messages = self::hasTag('imap')
                ? $this->getMessagesViaImap($this->email)
                : $this->getMessagesFromDb();
  
              if (empty($messages)) {
                  throw new \RuntimeException(sprintf('The message for "%s" was not sent.', $this->email));
              }
  
              foreach ($messages as &$message) {
                  if ($message['to'] === $this->email) {
                      $message['links'] = $this->parseLinksText($message['body']);
                  }
              }
  
              $this->messages[$this->email] = $messages;
          }
  
          // The debug messages may differ due to testing testing mode:
          // Drupal mail system collector or IMAP protocol.
          self::debug(['%s'], [var_export($this->messages[$this->email], true)]);
  
          return $this->messages[$this->email];
      }
docs: @param string $to
  
  @throws \RuntimeException
  
  @return array