Since Peppol was integrated with Odoo in v17, there have been many changes between versions. In v18, it is no longer possible to set default sending preferences. For your European B2B customers, the sending preference is set directly on Peppol due to legal requirements. In this video, you will find a workaround to set a dual default preference, via Peppol and via email, as is currently done in the new version 19.