I am using KDE 5.8.0 (frameworks 5.26.0) and kernel 4.7.6-1 on Arch linux with a 4k display on my laptop. Rescaling is working like a charm for almost every app.

Kmail, however, shows a partially blank message pane. It is the only app that I've seen has an issue with the rescaling. I have rebooted a few times now since the rescaling was put in place and it continues to have the issue. See: https://imgur.com/a/g5Ejr

Anything I'm missing? Many thanks!

EDIT: The same problem happens with Akregator, it seems. See: http://imgur.com/a/nPvms

Looks like a bug with the web view used by KMail and high DPI. Can you open a bug report on bugs.kde.org?