I use web-based outlook on a windows 10 laptop. McAfee came with the computer and I had to activate it in before I could delete it - I figured I might as well use it for the free-trial period. The anti-spam folder appeared in my outlook and started sucking in emails - even ones from people in my address book which is clearly a malfunction. I restored the emails and deleted the folder but it just came back when I went back into outlook. This time it was pulling in very important emails but the only way I knew it was by searching for them specifically. They weren't in the folder if I opened it but search found them. I uninstalled McAfee through windows. Folder kept coming back. Contacted outlook support. They said it was McAfee. Used McAfee's removal tool. Folder kept coming back. Contacted McAfee support. After two days and a total of 6 hours waiting for help, spent another hour and a half online with support. Folder STILL coming back. Support assures me that it isn't hiding any more of my emails, but how can I trust that? The folder shouldn't even be there with the program gone. Oh yeah, and McAfee support told me to contact microsoft.
I don't know what it was. They were not able to help so I didn't bother keeping it. As I noted in the question, I have a laptop running windows 10. I also have outlook on my desktop running windows 10. I also have it on my android phone.
I deleted McAfee from both computers and used the removal tool provided by McAfee to take off remaining program files. I never had McAfee on my phone. I don't have access to my desktop right now since I am working from home. I deleted outlook from my phone and reinstalled it - the folder is there.
The folder keeps recreating itself every time I open outlook on both my laptop (web version) and phone (app). There HAS to be a way to get rid of it.