Hello dear Manjaro fellows!
Since some days I have a really awful situation.
Aldo
I run a fully updated system here — Stable branch — but I am not seeing what you are seeing.
I do however notice that you’re running kmail from within kontact, whereas I am running kmail standalone.
Well, for that we would first and foremost have to discover what causes it in the first place. 
Maybe that is your clue? 
Perhaps you could start by telling us whether this is a machine with an Nvidia GPU and the proprietary driver. And then you could add when exactly you are seeing this behavior, because as you said, it does not occur when you are composing or replying. And just for giggles, you might also want to throw in whether this was an encrypted email or not.
Without any of the above information, we are as clueless as you are, and our crystal ball is currently over at the workshop for its annual maintenance. 
I didn’t even know maintenance was required. Mine straight up collapsed under pressure. Performance anxiety…
P.S.: I’m thinking fonts.
No, my machine doesn’t use proprietary drivers, I use an AMD graphics card with the drivers coming from manjaro.
The problem is, I don’t remember well, when I saw it for the first time because I don’t use this machine for mailwriting in the first place. And none of the mails are encrypted, because I don’t use any encryption at all.
I already tried to set back all the kmail options, but it’s still the same appearance.
Mine shattered itself too.
Maybe it’s an issue with whatever renders the preview. I’ll see if the Internet reveals anything, when I have a chance.
The sender could have encrypted them, and then it is up to kmail to automatically decrypt them again for you.
I can’t find much, but if I had to guess I’d blame the default font in use for previewing mail – it’s possibly damaged.
If we assume the sender(s) see no problem and it only occurs with the receiver (you), then the problem is local.
Another possibility is that a sender uses a font that you don’t have, and there isn’t a fallback – but this isn’t likely if, as you say, the mails are not from just one sender.
The font used for writing an email is likely different from that used when previewing a received mail. You could look through Kmail settings and see if there is an option to change the (preview) font.
Otherwise, I don’t know what else to suggest.
I should add that this is only speculation.
Regards.
It says “HTML Nachricht” on the side, so it’s an HTML email?
Looks to me like the HTML rendering is messed up. Can you test with an non-HTML email?
Moderator Note:
Topic title changed for consistency with similar issues in the last few days – using “garbled” as a keyword.
