In this tutorial on backups I've advised people to move their personal stuff to a seperate partition.
Email is also something that changes on a daily basis and something you don't want to lose in case of having to restore an image of your OS.
So you might want to move your Windows Mail files to a location of your choice.
Here's how to do that :)
Open Windows Mail and go to Tools > Options > Advanced > Maintenance > Store Folder.
You'll get a messagebox stating 'Your personal message storage is located in the following folder:'.
By default that is %USERPROFILE%\AppData\Local\Microsoft\Windows Mail\
using the change button you can browse to a folder of your liking and that can also be a folder on another partition.
My preference is to set it to the same partition as where you've placed your special folders :)
It works exactly the same in Windows Live Mail.