This is a nasty bug! Suddenly virtually every email I open in kmail appears blank. Worse - if I right-click and view source, there's nothing after the usual headers. It's as if the emails are only being partially downloaded, perhaps? I haven't touched my configuration recently. Strangely, just one email reliably appears and it's HTML.

Edit: Just tested some more. Even more bizarrely, it only seems to affect emails I've received since 9th October this year. Any email I open before that date is fine. But I read my emails every day so I definitely would have noticed this problem since 9th October! Also, I can read emails fine via the web interface and on my phone.

I'm using Kontact 5.1.3 on OpenSUSE Leap 42.1 64-bit. Please help!

Are you using POP3, IMAP, or are these emails in local folders?

I'm using IMAP.

In fact this no longer happens for new emails, but I still can't see the body of a certain number of emails. Is there any way I can mark them as not downloaded to force kmail to try downloading them again?

Install "akonadiconsole", navigate to the "Browser" tab, right click on the folder you want to clear the cache for, and select "Clear Akonadi Cache". Close akonadiconsole, run a new mail check in KMail, and the folder contents should be redownloaded.

Yep that did the trick. Thank you very much for your help einar!