Цитата из http://www.microsoft.com/windowsmobile/articles/directpush.mspx :

Windows Mobile Direct Push technology takes messaging to the next level by adding instant delivery. The Direct Push technology in MSFP means that Exchange Server sends Outlook messaging directly to your device over the air (OTA) through your wireless account. Like your desktop computer or laptop, your smartphone can offer quick e-mail flow and easy access to your calendar and tasks.

Это я к тому, что причем здесь вообще IMAP? Exchange работает же, вроде, по т.н. MAPI.