When I send mails or check for new mails, I see the small arrow button in the statusbar. In previous version, I could click on this and get a popup list seeing all currently running jobs and their progress including the possibility to cancel a job. The button is still there and changes up/down direction when clicking on it but the popup no longer appears.
Same problem here after updating to kmail-5.21.3. I think the progress popup is there but it is just not visible. If I enable "message preview pane below the message list" in Settings -> Configure KMail -> Appearance -> Layout and push the "Show Message Structure" button available via Settings -> Configure Toolbars, I can see a thin stripe of the progress popup in the border of the Message Structure pane. Then if change height of the Message Structure pane, I can see other parts of the hidden progress popup.
*** This bug has been marked as a duplicate of bug 460289 ***