[imp] How to set the IMAP path prefix in horde webmail 5.2?

[Quinn Comendant]

Does anybody know how to set the IMAP path prefix in the latest  
horde/imp? I'm getting the error when creating folders, "The mailbox  
'Sent' was not created. This is what the server said: IMAP error  
reported by server."

The imp_imap debug log shows that it is trying to create folders  
without the INBOX. prefix, which I think is the reason it is failing:

>> Mon, 08 Feb 2016 12:11:34 -0600
>> Connection to: imap://localhost:993/
>> Server connection took 0.0097 seconds.
S: * OK [CAPABILITY IMAP4rev1] Courier-IMAP ready. Copyright 1998-2005  
Double Precision, Inc.  See COPYING for distribution information.
C: 1 LOGIN quinn at example.com [PASSWORD]
S: 1 OK LOGIN Ok.
>> Command 1 took 0.0316 seconds.
>> CACHE: Using the Horde_Imap_Client_Cache_Backend_Cache storage driver.
C: 2 LSUB "" Sent
S: 2 OK LSUB completed
>> Command 2 took 0.0003 seconds.
C: 3 LIST "" Sent
S: 3 OK LIST completed
>> Command 3 took 0.009 seconds.
C: 4 CREATE Sent
S: 4 NO Invalid mailbox name.
>> Command 4 took 0.0003 seconds.
C: 5 LOGOUT
S: * BYE Courier-IMAP server shutting down
S: 5 OK LOGOUT completed
>> Command 5 took 0.0012 seconds.


I *am* able to create a folder if I manually include the IMAP prefix  
to its name, i.e. I go to Folder Actions > Create Mailbox and then use  
a folder name literally as "INBOX.Sent". The new folder is created  
with that name literally and "INBOX.Sent" is created on the server and  
is displayed in IMP's sidebar. =\

As far as I can tell, this is configured using the 'namespace' option  
in backends.local.php, but I have tried the following variations with  
no success:

     'namespace' => array('INBOX.')
     'namespace' => array('#INBOX.')
     'namespace' => array('INBOX/')
     'namespace' => array('#INBOX/')

How to set this correctly?

Thanks!
Quinn