looking for kmail in app menu
klick on the “Kmail holen …”
searching explicitly
so there is no kmail flatpak? As well as no korganizer, merkuro?
That’s right. Instead it’s “Kontact”, which includes KMail: https://flathub.org/en/apps/org.kde.kontact
The fact that KMail is advertised in the menu anyway when it won’t be available is a bug, though. So I’d encourage you to open a bug report about this at https://bugs.kde.org/enter_bug.cgi?product=krunner.
My solution is Distrobox. I install whatever I need in an Arch Linux box and export it to KDE-Linux. You can use software bundles from any distro you want.