how do i put emails in different subdir...

Hello there,
           
           I need to know if/how it is possible to have the email files
           generated and put into a subdirectory as related to the index 
           files, like this example:

           index_by_author.html
           index_by_date.html
           index_by_name.html
           emaildir/
                   email1.html
                   email2.html
                   email3.html
                   email4.html
                   email5.html
                   ...
                   
                   
           Anybody got any ideas?
           
Padraig Brosnan