[fouronnes3]

You make a good point but this is not a "strange assumption" by any stretch. You see Thunderbird has a "move" action. It allows you to move emails from a folder to another. Now somehow, this is apparently NOT implemented as:

1. Copy from source to destination

2. Verify copy has completed without issue

3. Delete from source

but... some other way? So when you try to move from a server folder to a local client folder by an innocent looking drag an drop, combine this with a poor network connection, a garbage tier legacy protocol like IMAP and decade old C++ spaghetti code, and you get this textbook 17 year old severity 1 bug that will never get fixed.

[thaumasiotes]

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.