Interesting, thanks for sharing the screenshot: makes everything easier to understand. I checked my account and I don’t have that folder in there. I suspect something is creating it. Can you make sure that none of the scripts on your site are sending emails to an address like
email@example.com? that +unwanted may be creating that address. If that’s not the case, check if there are rules that create the folder and dispatch there your emails.
If you don’t make any progress with this, please open a ticket so DreamHost staff can look deeper.