Instructions on how to move the email email and attachments to your storage card instead of main memory:

1. Create a 2 folders on your storage card.
\Storage Card\Mail\
\Storage Card\Mail\Attachments\

2. Modify HKEY_Current_User\Software\Microsoft\MAPI. Create 2 new string values. The first should be called PropertyPath and should have a value matching the mail folder you created. The second should be called AttachPath and should have a value matchings the attachments folder you created. (ie. \Storage Card\Mail and \Storage Card\Mail\Attachments)

You'll need to restart your phone and resync. Make sure to delete the files off the old locations in main memory as they don't get deleted automatically.