The new version of Mustangproject, 2.10.0, released today, December 30th 2023, adds write support for XRechnung 3.0.1, which becomes mandatory for german B2G invoices as of February 2024 (issue #343). Users will need to add email addresses for also for tradeparties themselves, not only for their contacts.
Additionally two minor issues were fixed, concerning PDF/A3 input from inputstream and the commandline is now again capable opf producing Factur-X 1 files, closing issue #354.