Good tip:
Migrating Mail from Lion to Mountain Lion leaves behind a folder containing previous attachments.Attachments in Lion were saved to ~/Library/Mail Downloads, but under Mountain Lion, Mail is now sandboxed, and the new path is ~/Library/Containers/com.apple.mail/Data/Library/Mail Downloads.
During the upgrade, the folder contents are copied from the old location to the new, so the old location can be deleted to save some space.