as a rule of thumb, its always a good idea to keep the email folders for all email programs in a separate partition on the disk. this way, if the system partition crashed, the email partition will be safe-rrr. in my opinion, if the folders and files are still physically on the disk but the master file table has become corrupted, then the MFT will make it look like the folders are gone. executing a check disk will re-index the MFT.