[psycho_maniac]

Create the following folders on your Storage Card.

\Inbox
\Inbox\Attachments
Now browse to \Windows\Messaging (SmartPhone) or \Windows\MAPI (PocketPC)
If you want to save all of your existing mail, move everything in this directory to the new ones on your Storage Card.
Now fire up your registry editor and navigate to the following.

For WM5 Phones: [ HKEY_CURRENT_USER\Software\Microsoft\MAPI ]

For WM6 Phones: [ HKEY_LOCAL_MACHINE\System\Inbox\Settings ]

Then create the following STRING VALUES.

Name: AttachPath
String: \Storage Card\MAPI\Attachments

Name: PropertyPath
String: \Storage Card\MAPI
Reboot your phone and then goto your inbox. If you can see all of your mail then mission accomplished. Double check your attachments as well.