This issue is not new in KMail 5.7.3, it was introduced in one of the previous version: If you have scrolled to the middle of a mail in the preview window, and then use the spacebar to jump to the next unread mail, the preview window remains somewhere in the middle of the mail instead of jumping to the beginning of the new mail. Was working before (Kmail 5.6.x)
Thank you for reporting this issue in KDE software. As it has been a while since this issue was reported, can we please ask you to see if you can reproduce the issue with a recent software version? If you can reproduce the issue, please change the status to "REPORTED" when replying. Thank you!
After 4 years, this is now fixed