First thing you need to do is create a couple folders on your storage card.The first folder should be \Storage Card\Mail\ and the second folder should be \Storage Card\Mail\Attachments\.
 
Once these are created, you'll need to edit your registry.
  • Open your favourite Windows Mobile registry editor
  • browse to HKRY_Current_User\Software\Microsoft\MAPI and create 2 new string values.
    • PropertyPath and should have a value matching the mail folder you created
    • AttachPath and should have a value matchings the attachments folder you created
So PropertyPath = \Storage Card\Mail and AttachPath = \Storage Card\Mail\Attachments
 
Enjoy the new free space:)