Postmark is releasing a new feature that allows customers to add and retrieve custom metadata for emails sent through their platform. This feature enables users to identify emails throughout their lifecycle, set unique IDs on a per-message basis, and associate this data with external records. With the release of custom metadata, Postmark is expanding its capabilities to be more customizable and powerful, allowing developers to integrate it into their applications in new ways. The feature includes limits such as field name and value length restrictions, but provides flexibility for users to implement their own unique solutions.