If you have a Windows Mobile 5 device that was shipped before MSFP (Microsoft Feature Pack) was released then you'll probebly find your OEM has released a ROM update for your device.
If you want to make use of Push Email like the Blackberry range of devices, then you will want to install this update.
Most major providers were quick to release ROM updates. I recently updated an Orange M3100 with the MSFP ROM update and it worked just fine. I only have another 1199 devices to do! I think I will delegate!
If you too have an Orange M3100 you can get the update from here: http://www.orange.co.uk/business/downloads/SPV_M3100_ROMUpdate_0407.exe
Release notes for the M3100 can be found here: http://www.business.orange.co.uk/servlet/Satellite?pagename=Business&c=OUKPage&cid=1044134915191
Microsoft left it to the OEM's to provide the update, so you will need to check with your provider if they have one.
Note: One major requirement for push email to work via WM5/6 is Microsoft Exchange Server 2003 SP2 or later is required.