[Dovecot] Question about "target is inferior" type of message

[Michael Durket]

We're converting UW-IMAP users here to dovecot. Some of them have seen  
the error message
issued when Thunderbird clients try to delete folders and I understand  
what's going on (from
previous postings) that Dovecot doesn't support mbox subfolders within  
folders.

What puzzles me is that many of these users already have folders  
within folders (I've found
typically 2 levels of nested folders within their main Mail  
directory). And they're not complaining
that they can't read the messages in those folders. If UW-IMAP  
supposedly never supported
this for mbox format, and Dovecot doesn't support it, how is this  
possible under Dovecot?
(We don't have any maildir format mailboxes here at all).