The collaborative process with Posteo actually uncovered some bugs in LibreJS, which are now being fixed. Once they are fixed and the site is re-tested, we will be able to announce Posteo's full compatibility with LibreJS. We just didn't want to wait to publicly thank them for what they have already achieved!
Stay tuned for more exciting announcements about Posteo and other sites soon! And if you'd like to send a social media "Danke schön" to @Posteo_de, we think they'd appreciate it. (Just make sure not to use proprietary software while doing it.)