By default, all emails are stored in the hMailServer data directory, under C:\Program Files\hMailServer\Data. In some cases, you might want to change this to a different path. For example, if you have more disk space on another drive.
NOTE:- This will only work if the diagnostic tests show that 'Relative message paths are stored in the database for all messages' - (second last test). If this is not the case then head to the forum and ask how this can achieved.