Synchronising email subfolders on Windows Mobile
I am asked very often by users new to ActiveSync, how do you synchronise folders other than the Inbox: is it possible to synchronise Sent Items and subfolders of the Inbox? The answer is yes.
Here are some instructions for both Pocket PC and Smartphone devices.
Pocket PC
Launch the Messaging application from the Start menu:

Select Outlook Email from the list of accounts.
Tap on the Menu button and select Tools, Manage Folders:

Your Outlook folder structure will be displayed, select which folders you wish to be synchronized to the device.
Smartphone
Launch the Messaging application from the Start Menu.
Select Outlook Email from the list of accounts.
Press the Menu button and select Folders:

Your Outlook folder structure will be displayed, select which folders you wish to be synchronized to the device.