In February 2026, Viktor Hesselmann enhanced the Symfony Mailer by upgrading the Mailjet transport to support inline attachments with custom Content-IDs, addressing a key need for improved email rendering and organization. Working within the symfony/symfony repository, Viktor implemented this feature using PHP and Symfony, focusing on robust API integration and backend development. He ensured reliability by adding comprehensive tests that validated both custom and default Content-IDs in the payload, and resolved a Content-ID propagation bug to guarantee correct rendering. This work improved the construction of Mailer payloads, increased test coverage, and reduced renderer-related issues for end users and partners.