Thanks for a bug report. i tried to reproduce this with current git master, which is pretty much the same as to-be 3.18.1, and the move on GMail server using IMAPx works just fine.

My steps:
a) create folder Inbox/test
b) create folder Inbox/test/test2
c) copy one message into Inbox/test
d) move one message from Inbox/test into Inbox/test/test2 using drag&drop

The message is moved, not copied. Furthermore, the GMail server supports
UID MOVE command, thus it takes care of the move, not the evolution. When I run evolution from a command line like: 
   $ CAMEL_DEBUG=imapx:io evolution
then after the step d) I see the move command there, which looks like:

> A00051 OK [READ-WRITE] INBOX/test selected. (Success)'
> [imapx:A] I/O: 'A00052 UID MOVE 1 INBOX/test/test2'
> [imapx:A] I/O: '* 1 EXPUNGE
> * 0 EXISTS
> A00052 OK [COPYUID 58 1 1] (Success)'

which is just as it should be.

There is a related issue fix [1], committed for 3.17.92, but as you have filled that you use 3.18.x, then you should have the fix included already. That is in the evolution-data-server.

Could you verify your evolution-data-server version and paste it here, please? Could you run the evolution from a terminal with the CAMEL_DEBUG as shown above, and copy&paste the command being run from the time of the message move, please? Maybe you do other steps, and they behave differently?

[1] https://git.gnome.org/browse/evolution-data-server/commit/?id=73d0803

I've tested some more, and it seems I cannot duplicate it under X11, but I can under Wayland.

The text written in the status bar under X11 is "Moving messages to folder ...", the text written under Wayland is "Copying messages to folder ..."

Might be a GTK+ drag-and-drop problem?

I'll get CAMEL_DEBUG as soon as I run Wayland again.  Not today.

(In reply to Berend De Schouwer from comment #2)
> Might be a GTK+ drag-and-drop problem?

Under Wayland, I see. I agree with the above statement, it really might be something wrong in a lower level, like the gtk+. I'm moving this there.

No need to capture the CAMEL_DEBUG log, the Wayland thing explains it.

Yeah, I'd agree; see https://bugs.freedesktop.org/show_bug.cgi?id=91944

Yeah exactly, there is currently no concept of "actions" during DnD operations, I made GTK+ default to "copy" as that's less prone to result unexpected behavior or data loss. 

There's already wayland proposals around adding the missing actions, plus branches for mutter and GTK+ that'd make things just work.

*** Bug 735849 has been marked as a duplicate of this bug. ***

*** Bug 758392 has been marked as a duplicate of this bug. ***

*** Bug 758745 has been marked as a duplicate of this bug. ***

this has landed now

Does this mean it has been corrected?

When will it be available?

(In reply to Leo from comment #10)
> Does this mean it has been corrected?
> 
> When will it be available?

In Gtk+ 3.20 or its prereleases, 3.19.x

This seems to be fixed in Nautilus, but not evolution

nautilus 3.20
evolution 3.20.2

evolution always says 'copying'

that needs to be analyzed on the evolution side first.

(In reply to Matthias Clasen from comment #13)
> that needs to be analyzed on the evolution side first.

Apologies.  I originally marked this bug against evolution.

Works for me, evolution-3.20.3, the only difference is that the default action under X is move, while the default action under Wayland is copy, which can be changes using the Shift, respectively Control, key while dragging the item.

@mcrha: Can you please make dnd default to move again? Copying mails is quite uncommon, moving them is not.

(In reply to Christian Stadelmann from comment #16)
> @mcrha: Can you please make dnd default to move again? Copying mails is
> quite uncommon, moving them is not.

I've filed a separate bug: #768013

*** Bug 756656 has been marked as a duplicate of this bug. ***