KMail2 support for KWin Window Rules is completely broken. It does not respect any of the settings entered for its window via Window Special Settings (or Application Special Settings) dialogue, e.g. the ones on the Size & Position tab.
$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 17.04
$ dpkg -l kmail
||/ Name Version Architecture Description
ii kmail 4:16.12.3-0ubuntu1 amd64 full featured graphical email client
FYI: The bug is still here after the Plasma 5.10.2 / KDE 5.35.0 update.
I can confirm this, too - Manjaro testing with 5.15.3 currently
In newer versions, kmail (and any other application) should remember its size and position automatically, no need for windows rules. At least that works for me.
Can some of you please confirm that it still happens with a recent version of Plasma and Kmail?
it's gotten much more rare for me that it doesn't remember the correct size and position, that much i can confirm. I'd say the instances where it is launched in the wrong place are mostly after crashes or forced reboots or the like, but i'm not 100% positive.