its not possible trough IMAP. But i use Z-Push for this thing. Z-Push is OpenSource and devloped in PHP. It lives "between" some "Backends". Backends can be IMAP, CardDAV, CalDAV, LDAP, Zarafa or other Servers. On the other side is a Mobile Phone with Exchange ActiveSync Protocol. Then you can sync and get Push-Mails also from a IMAP Server and the other Backends to your Phone. Basicly it is developed for the Zarafa Server, but it also works with the Backends above. I use it with my Windows Phone 7.5 and it works very good.