For those who are still getting the following email from Outlook:
"Subject: Folders in your mailbox have name conflicts
One or more folders in your mailbox have name conflicts with others or system reserved names
Folders with these names can't be downloaded by your IMAP e-mail program.
Junk
Please connect to your mailbox using your Web browser and rename those folders. Once renamed, those folders will be displayed in your IMAP e-mail application correctly."
The problem is that Microsoft has changed the expected name of the "Junk E-mail" folder to "Junk Email". The iPhone and iPad don't know to look for that, and so when you mark an email as junk in IOS, it creates a "Junk" folder in your Outlook, and moves the email into it. The email will never be deleted unless you go into the Outlook app or the Outlook website to do it manually. Microsoft doesn't like this folder, and sends you an email to complain. Although the email sounds like tragic news, it doesn't actually seem to be a major issue.
The easiest solution is not to use the IOS's "Mark as Junk" feature. A more complicated method to forcibly rename the Outlook folder can be seen above from mattj1856, but requires that you have a Windows computer with the Outlook app installed, and may cause issues with Outlook in the future.
In addition we could complain to Apple for not allowing us to change the expected IMAP name (although other clients don't seem to allow it either) and/or complain to Microsoft for once again doing their own thing in the face of international standards.