I had a similar problem with my Outlook 2007.
I spent nearly 2 weeks trying all sorts of remedies I read about on the web.
There are multiple causes that can show these same symptoms, so it is hard to know what is causing it.
- I tried creating a new .pst file for my inbox
- Checking for a "Rule" (in Outlook, Tools > Rules and Alerts) that tells all mail to go into Junk.
- I created a new Profile.
- Thinking it might have been caused by a Microsoft Security upgrade, I restored back to my oldest backed up configuration using Windows Restore (which actually caused more problems).
You could try all these things.
None of them worked for me, however.
Eventually I realised that my antivirus program was not working properly (Norton in my case) I got Norton Support (Symantec own Norton) to uninstall and reinstall it and that fixed the problem with all my inbox mail going into the Junk.
I believe that Norton must have a default that if it cannot verify mail as "clean" it diverts it to the Junk folder. Since Norton was not working correctly it was failing all my mail messages, but the Norton Outlook extension that drops unvalidated mail into the Junk folder was still working.
That is my surmise.
Note: I use popmail not IMAP, but I doubt if that makes a significant difference in this particular case.