Increase the size of the top split of the horizontal split

[GretaD]

Is your feature request related to a problem? Please describe.

The part of mail list is too small and you cannot see more than a couple of messages, now it is approx. 40%, which is too few.

Describe the solution you'd like

Make the mail list part wider

Describe alternatives you've considered

No response

Additional context

Ref #9462