IMAP is a very 'flexible' standard, to say the least, so it is difficult to support the multitude of available servers, each configured slightly differently. We strive to support as many servers as we can, but some configurations are simply incompatible with our methodology or result in a poor user experience.
In your case, the incompatibility stems from the fact that the mail server does not confine the IMAP client to the mail folder. This causes our IMAP client to crawl through and synchronize the entire folder hierarchy instead of just the relevant mail folders. With a sufficiently large and complex folder hierarchy, this is enough to cause significant user pain in the form of wasted time, memory, and bandwidth.
Some mail clients get around this issue by allowing the user to specify a new root folder. Unfortunately the WM 5.0 IMAP implementation does not support a user configurable root folder.
Configurable IMAP folders (root, deleted, sent) have been requested and considered in the past. But due to the vagaries of the software development process, this feature has slipped through the cracks. We are always working to empower our users, and I hope that we can address this issue in a future release.
SDE, Outlook Mobile