Kidding, although I do this if I’m on the machine, I use claws. claws is just fine.
Kube appears to be a rather new project aiming on replacing kmail in the long run (whith slowly extending it’s feature set). Kmail itself is rather complex and has an unsteady past. I remember the mobile kmail interface beiing available for Maemo/MeeGo years ago but I have the impression those got depracted later.
There doesn’t appear to be an email client available on kde plasma mobile yet either!
As a user of KMail for two decades, it is now slow and unreliable. It was a good plain, reliable, program back before the client:server fairies got to it. Sadly it’s one of the many programs that never recovered properly from KDE’s Triassic/Quaternery mass extinction event.
As a user of KMail since 2008, it has always served me well, and still does. Very reliable and configurable. From time to time I’ve tried other email-handlers that has more bells and whistles, but always come back to KMail - with a sigh of relief. Just my 2¢.
Claws Mail 4.0.0 looks good on Ubuntu 22.04, installed right now from Synaptic and will test. Installation went fast and flowless.
edit: email server setup no problems, works fine.