[imp] Bizarre folder creation failures

[System Administrator]

I recently hooked up the latest and greatest of the non-CVS 
Horde+Imp+goodies. Everything works marvelously for the vast majority of 
my users. Except for a few, and this seems to strike with no rhyme nor 
reason.

Simply put, some of my users simply cannot seem to save their outgoing 
mail into the sent-mail. In fact, the system won't even create a 
sent-mail folder for them.

Now, when *I* log in as them, and send out mail, it works flawlessly, 
creating the folder and saving my outbound message. But even with it 
created for them, they still cannot save outgoing messages.

They have NO problem manually creating folders on their own. And the 
trash folder works. Just not sent-mail.

It's obviously not a problem creating folders, as they can do it 
manually and I do it automatically when logged in as them. It strikes 
across the board. I've had users with OSes ranging from MacOS9, to 
almost all variants of Windows, and even one Linux user, all experience 
the same problem. They are all dialed directly into my system, or are 
DSL or wireless customers (hence directly on my network). And it doesn't 
matter what browser they use. I've had them try everything from Firefox 
to Opera to Infernal Exploder, nothing works.

Any ideas? Or do I bite the bullet and install something like O-mail for 
these few problem customers?