Randomly, mails go missing from my inbox. Obviously, I have no way to reproduce that reliably, and I don't know whether there are any logs that can be used to figure out what might have happened and when...
Using package kded5 on kubuntu 20.10 I'm experiencing similar issues. Using postresql for pim and kontact (but not using either app in user mode.) Have 16GB Ram, 64G swap, and a pair of nvidia 970 cards to produce a 4k screen. My normal working setup is to have kmail in one virtual screen and dolphin in another. If I leave the system up for more than a day or so, one of three things will happen: 1. the system crashes outright 2. the toolbar disappears (so I reboot from another term) These seem to be more related to the presence of an idle dolphin so mainly here for context. My main issue here is that it comes back up with ancient attachments (like to usb drives.) 3. kmail goes wonky and clears out all the email and all the folders. Importing a previously-saved mail works, and then after about ten seconds, kmail clears all of the messages and folders again. Main folder (Local Folders) is set to never expire. (The update nailed me on that one.) All the other folders inherit so should never expire either. The local folders is set to store to ~/.local/share/local-mail but the mail is actually stored in ~/.local/share/.local-mail.directory. That made scripting a backup problematic. I don't know if this behavior exists without dolphin running (because I'm trying to avoid it, not re-create it.) If need be I can get a ps next time it happens so we can see what else is causing corruption or memory leak or whatever. --- If we can't identify and solve the suddenly missing mail issue, can we create a backup journal somewhere, so it's easy to restore or at least not lose a month's worth of prospective work contacts?