RIM has never fully supported IMAP servers in their Blackberry Internet Service(BIS) formerly Blackberry Web Client(BWC). Notably BIS does not recognize the NAMESPACE command from IMAP servers. The vast majority of IMAP servers place all user folders in a single parent folder(usually INBOX) therefore the Sent and Trash folders are actually INBOX.Sent and INBOX.Trash. Unfortunately BIS only recognizes Sent and Trash in the base directory.
This script sits between the BIS and your IMAP server. The proxy then makes INBOX.Sent and INBOX.Trash appear to be Sent and Trash. This enables BIS to save a copy of your sent emails into INBOX.Sent properly and a copy of deleted emails into INBOX.Trash properly.
Additionally this script allows you to use a non standard folder as your inbox as well.