I have the exact same problem. It seems to have started up recently. Looking at your post date of 3/1/2015, mine might have started a little bit later, but nonetheless around the same timeframe. I have tried deleting this 'shadow' folder, and it goes in the trash, and I empty the trash, and I think everything is great. Until the next time I start up TB, and it shows up as a Subdirectory of "Inbox". I use IMAP, and I check on the server side and see it there. And when I delete it from TB. It disappears from the server side. I empty the trash. I keep outlook closed for many hours, check the server side and it's "clean", as soon as I start up Thunderbird, within seconds, the 'shadow' (Inbox->"Inbox->Trash") pair of directories show up. And then it shows up on the server side as well. I have quite a number of email accounts/profiles, and it seems this happens to all but 'one' of them. Not sure what's magical about this one other profile. Otherwise, the other 10+ accounts ALL have this problem now. Some were created years and years ago, and some were created within the last year. I'm on a Mac. At first (and still suspect) I thought it might have had to do with the 'Server Settings-> When I delete a message: -> Move it to this folder" setting, but I checked and all the old accounts and the new account has the same setting. It sets it to "Move it to this folder: "Trash on xxxx@DDDD.com" of the corresponding account. And on the 'new' account, it is set to "Choose Folder". I've left it alone. For fear that if I 'choose' the folder to be like I've set up all the others, that this will start creating the 'Shadow' "INBOX->Trash" folder under the 'Inbox' of all my accounts. I wish there was a way for me to 'reset' the selection on one of my other accounts to "Choose Folder" and see if it helps 'fix' the problem. Nonetheless, it seems to be a weird bug. Screen shots attached: 1) shot of problem situation, 2 what it should look like (aka when I delete the Shadow INBOX->Trash before it reappears), 3) settings on problem accounts/profiles, 4) setting on newly created no-problem account/profile) Hopefully, this provides a bit more color to my version of what seems to be the same problem. Thanks.