Message Stream ID is <strong>optional</strong>. By default <strong>outbound</strong> (Default Transactional Stream) will be used. More information can be found in our <a href="{bbackup_94402fb9ba80f392e0976ece85f92b2b876bfcaed8d83250cc06083f7139c7d9}s" target="_blank" rel="noopener noreferrer">Postmark documentation</a>.