[exim] spool directories

I have 3 mail servers one pop3 server and 2 smtp servers. My smtp servers
place everything that is in the mail que in /var/spool/exim/input/

however my pop3 server places the mail that is in the que in
/var/spool/exim/input/ and in the input directory there are numbers 0-9 and
a-z and A-Z , exim seems to be separating the spools according to the first
character of the que name. I do have exim setup to hash out the username of
the user that gets email for example if joe was sent an email it gets sent
to /spool1/j/o/joe and in joe there are new/ cur/ tmp/ but I am not sure why
my mail que gets hashed out the way it does. Any help would be greatly
appreciated.