Improve comments for sending to multiple addresses

Below is the code that ended up working [ where MODULENAME is the name of the module ]

It is called like this:
$mailManager = \Drupal::service('plugin.manager.mail');
$result = $mailManager->mail('MYMODULE', $template_key, $recipient, $langcode, $params, $scaf_from_email, TRUE);

where $recipient failed if it was something like "a@a.com, b@b.com"

I had to insert the lines in the build() function

$to = $email->getParam('to');
$address = $this->parseAddress($to);
$email->setTo( $address )

;

Here is the whole code:

/**
 * Defines the Email Builder plug-in for user module, used by symfony_mailer
 *
 * @EmailBuilder(
 *   id = "MYMODULE",
 *   common_adjusters = {"email_subject", "email_body", "email_to"},
 *   import = @Translation("Update messaging"),
 * )
 */
class MYMODULEmailBuilder extends EmailBuilderBase {
  
  use MailerHelperTrait;
  use TokenProcessorTrait;
  
  
  /**
   * Saves the parameters for a newly created email.
   *
   * @param \Drupal\symfony_mailer\EmailInterface $email
   *   The email to modify.
   */
  public function createParams( EmailInterface $email, 
                                $body = NULL, 
                                $sender = NULL, 
                                $recipient = NULL, 
                                $subject = NULL, 
                                $params = NULL) {

    $email->setParam('params',$params);
    $email->setParam('to', $recipient);
    $email->setParam('subject', $subject);
    $email->setParam('body', $body);
    $email->setParam('from', $sender);
  }
  
  /**
   * {@inheritdoc}
   */
  public function fromArray(EmailFactoryInterface $factory, array $message) {
    $sender    = $message['params']['from'];
    $body      = $message['params']['body'];
    $recipient = $message['to'];
    $subject   = $message['params']['subject'];
    
    $params    = $message['params'];
       
    return $factory->newTypedEmail($message['module'], $message['key'],
                                    $body,
                                    $sender,
                                    $recipient,
                                    $subject,
                                    $params );
    
  }
  
  /**
   * {@inheritdoc}
   */
  public function build(EmailInterface $email) {
   
    $to = $email->getParam('to');
    $address = $this->parseAddress($to);
    $email->setTo( $address );

    $email->setSubject($email->getParam('subject'));
    
    // This is how we pass the params to the template
    $email->setVariable('params', $email->getParam('params') );
  }
  
  
  /**
   * Taken from symfony_mailer\MailerHelper
   * 
   * {@inheritdoc}
   */
  public function parseAddress(string $encoded, string $langcode = NULL) {
    foreach (explode(',', $encoded) as $part) {
      // Code copied from \Symfony\Component\Mime\Address::create().
      if (strpos($part, '<')) {
        if (!preg_match(self::FROM_STRING_PATTERN, $part, $matches)) {
          throw new \InvalidArgumentException("Could not parse $part as an address.");
        }
        $addresses[] = new Address($matches['addrSpec'], trim($matches['displayName'], ' \'"'), $langcode);
      }
      else {
        $addresses[] = new Address($part, NULL, $langcode);
      }
    }
    return $addresses ?: [];
  }
}

It is built explicitly:

$recipient = from_email . ',' . $partner_email;
...
$mailManager = \Drupal::service('plugin.manager.mail');
$result = $mailManager->mail('MYMODULE', $template_key, $recipient, $langcode, $params, $from_email, TRUE);

This is how it worked with swiftmailer, mailsystem and hook_mail, and the comment above parseAddress() basically states that.

I certainly can move the Address part up to the caller.

However, my point was, when I ported the code this was not documented. If it had not been for that patch in the issue about the legacy mailer, I would not have found this. Other people going through this process may face the same issue.

For that reason, it should be at least documented somewhere (like in the "how to port from swiftmailer" article), or even better, catch this somehow in the baseclass so that it is transparent.

comments make it clearer. Reviewed and tested by me. Thank you.