SUMMARY

Kmail does not remember toolbar positions and lock status configuration after message window close and reopening with "New message".

Configuration of each toolbar, e.g. the main or the html toolbar are remembered over window close and new message (icons, text, elements).

STEPS TO REPRODUCE
1. Download kdeneon iso and start as Guest
2. Install kmail AND libyxing3 via apt
3. Launch kmail
4. open kmail message window by 'new message'
5. Make the html bar visible and unlock toolbar positions
6. move the bar on the bottom of the window (e.g. by 'Orientation' -> 'bottom' in the context menu)
7. Close the window and re-open it by 'new message'

OBSERVED RESULT
Toolbar positions and lock status are reset after closing and re-opening message window.

EXPECTED RESULT
Message window should remember both, toolbar positions and lock status.

SOFTWARE/OS VERSIONS
This was tested on two different systems with new configurations: 
Arch linux (recent update today):
Operating System: Arch Linux 
KDE Plasma Version: 6.0.2
KDE Frameworks Version: 6.0.0
Qt Version: 6.6.2
Kernel Version: 6.6.22-1-lts (64-bit)
Graphics Platform: Wayland
Kmail: 6.0.0 (24.02.0)

kneon user edition (in VM) downloaded as of today (20240321)
Operating System: KDE neon 6.0
KDE Plasma Version: 6.0.2
KDE Frameworks Version: 6.0.0
Qt Version: 6.6.2
Kernel Version: 6.5.0-26-generic (64-bit)
Graphics Platform: Wayland
Kmail: 6.0.0 (24.02.0)

Additional Information:

I think this bug was already present in previous versions of kde before the big update to 6.
Also happens if only the main toolbar is moved to the bottom and toolbar positions are locked after closing the window.