I checked the @gajim issue tracker for PKIX over Secure HTTP (POSH) support and it looks like it was implemented already 5 years ago: https://dev.gajim.org/gajim/gajim/-/issues/9083

But I can't find the setting in the current #Gajim version and didn't managed to connect to my #xmpp server so far which uses "PKIX over Secure HTTP" for ssl verification.

#followerpower, any ideas?

Support for PKIX over Secure HTTP (POSH) (#9083) · Issues · gajim / gajim · GitLab

This makes it possible to use a custom domain with a XMPP hoster without having to accept a self-signed certificate. https://datatracker.ietf.org/doc/rfc7711/?include_text=1

^GitLab