Kmail keeps crashing in Kubuntu 18.04
 

I have just installed Kubuntu 18.04
Kmail keeps crashing every time I click on a message.
It opens ok, all folders are there but as soon as I click on a message to see the content, the Kmail shuts down.
It's a Kmail version 5.7.3, with KDE Frameworks 5.44.0
I've never seen Kmail doing that before.

Try this:

Start Kmail from the command line and see whether it throws any useful error messages to the terminal. If it does, post them here, being sure to surround any terminal output with "code" tags, which become available when you click the "Go Advanced" button beneath the "compose post" window.