Maybe a day I'll try to work with my Smartphone's registry and this tip is really useful to try:
In order to get Outlook Mobile to store your emails on the SD Card, you've to set the registry key HKCU\Software\Microsoft\MAPI with these values:
AttachPath string “\\storage card\\mail\\attachments” (no quotes)
PropertyPath string “\\storage card\\mail” (no quotes)
Thanks to Mike Poulson... bookmarked!