Objection: I think it would be premature to bet mutt's features and port nontrivial amounts of code on some 0.X pre-release of some library -- where the latest release announcement (BTW, 0.4) mentions "Please test whether Libidn2 solves your IDNA2008 needs!".
Feel free to ask again once libidn2 is stable. -- Matthias Andree
