A few incoming mails, when opened, cause kmail to go to the "loading file" screen, and it simply remains there for however long I leave it. At that point, no other email can be selected. It is necessary to close the kmail client and then go to a shell and kill a number (usually about six) spawned kmail processes.
At this point the client can be restarted and runs more or less normally. If I land on the errant mail, even the attempt to preview will lock up the client. Hence I can't delete it and I can't actually select it to send it in. Fortunately this is not the inbox--filters move it just fine. If I select the entire folder, no operation (other than opening it and locking the client) is possible. It does not sort, does not clean, and does not export.
These messages seem to be related to passing information back and forth with the system browser. They are invariably confirmation messages for a web course I'm taking."
Will accept instructions for how to extract the mail or folder, or how to get into the database, or whatever, if it helps.
This is on kubuntu 16.10
Here's the info from the client:
KDE Frameworks 5.26.0
Qt 5.6.1 (built against 5.6.1)
The xcb windowing system