Working with Message-IDs

Emails can contain a unique ID known as a message ID. As of Swift version 3.1.4, Swift will generate this ID for you on a per-send basis (i.e. when you send a message an ID is created for it). If you want to get the message ID yourself then you should override what Swift does and call the generateId() method of Swift_Message:

$message = new Swift_Message( ... ); //as normal
 
foreach ($recipients as $address)
{
  $id = $message->generateId();
  echo $id; // something@domain.tld
  $swift->send($message, $address, $you);
}

generateId() accepts an optional string parameter which gets included in the overall message ID if passed.

Knowing the message ID before the message is sent comes in useful when you are sending to a mailing list where you will detect bounces/track message statuses later.