This task still hasn't completed.
"Checking for Mail…" comes and goes as one might expect, and normal mail functionality seems to work fine. Connection Doctor shows nothing strange.
However, this task has been going on for days, and moves very slowly—I've left the computer on all night and it only chews through a few thousand, occasionally stopping with an error. If I restart Mail.app (it takes a bit to quit), it eventually comes back up with the same 79,718 messages needing to be moved.
Is there any way to terminate this process? I'm inclined to try another tool at this point, but I'd like Mail.app to stop trying to move these messages.