burngreave.net

You can create subfolders in IMP from the folders menu. You can the set up filters to move into folders automatically, based on things like who it was from, or sent to, or particular words in the subject or body

Unfortunatley they cannot contain other folders...

If you need to be able to create subcontainer to put other folders in like

work/project1
work/project2
personal/joe
personal/jane

You can do it like this

I just realised that you can create folders from Horde Imp that will have
subfolders by naming them like mickey/finn
 
this will create a filesystem folder and place a mailfolder inside it so you
can move your e-mail to mickey/finn ... etc