The best way to get developers to fix these kind of crashes is to open the crash dialog you get when KMail crashes (or click the sad face icon in the system tray, if you don’t get the dialog) and ask it to submit an automatic crash report to KDE.
Because it looks like you have a very clear understanding of the issue, and a simple reproduction, the second best way to get developer to fix these kind of crashes is to create a bug report at bugs.kde.org
While it is OK to post these things to the Discuss forum here - because you have such a good report and it doesn’t look like you need any help in understanding the problem - it is much less helpful to post it here than in bugs.kde.org.