From the Microsoft Knowledge Base article:

http://support.microsoft.com/kb/287574

"Using Outlook with IMAP your Sent Items, Drafts, and Deleted Items folders cannot reside on an IMAP server. Outlook requires that all special folders exist in the default store. IMAP cannot act as the default store because it cannot contain the non e-mail special folders.

Deletion works through marking items for deletion, and then purging all items marked for deletion in a given folder. There is not a Deleted Items folder because moving an item in IMAP is inefficient. The source item is not immediately deleted due to the two-phase deletion model. This requires that you double the storage requirement for that message."