FlexMail is at
www.webis.net. It supports IMAP IDLE which is as close as you can get to push email without an exchange server. It also supports Exchange accounts by directly using the ActiveSync folder. IMO it the best 3rd party email client for WM.
To answer the OP....Personally, if you can afford the $9/mn, I would highly recommend getting a true Exchange server address and automatically forward any of your other email accounts to that address. There are many companies out there offering single exchange accounts.