Well, interestingly, Thunderbird insists that that is exactly the implementation they're already using.

(see https://bugzilla.mozilla.org/show_bug.cgi?id=462156#c69 )

Of course, it isn't; they're relying on an unstable proxy for "copy has completed without issue" in preference to just checking whether that happened or not. Several rather angry users have pointed this out. It doesn't seem to have sunk in.