were the mails actually downloaded to Thunderbird before the move? I am guessing they are pending the completion of the copy by downloading their content from the server.

Yes the messages were completely downloaded.

What platform is this on? I've seen this no entry symbol, or something very like it, in some Gnome (read: Linux) configurations.

In my case it was an alternative to the ambiguous X symbol meaning delete (as opposed to the use of X for "close".)

Hi - its Thunderbird 38.2.0 on Mac OS X 10.10.4

Cheers Neil

I don't know of a definitive list of icons. There is an old one and it doesn't cover all the various permutations across platforms and to add further confusion, you may get different icons with various themes. I generally try to find colourful themes that match the symbols elsewhere in my LMDE desktop.

Having said that, this page appears to say your icon means the message has been (or is or will be) deleted, which is somewhat distressing if it's being shown in the destination POP account folder. However in the originating IMAP folder it would seem to make perfect sense.

Thanks for the Page link Zenos - very useful.

Zenos said

I don't know of a definitive list of icons. There is an old one and it doesn't cover all the various permutations across platforms and to add further confusion, you may get different icons with various themes. I generally try to find colourful themes that match the symbols elsewhere in my LMDE desktop. Having said that, this page appears to say your icon means the message has been (or is or will be) deleted, which is somewhat distressing if it's being shown in the destination POP account folder. However in the originating IMAP folder it would seem to make perfect sense.