[Exim] mail server for large sites

Hi

I've been asked to design/setup a mail server for
large customer base >500K , I'd like to split up
the mailboxes across several machines, this I
belive I can do using exim, and doing a lookup in
the mysql db for delivery. However this is where I
get stuck (in theory since I have not practiced it
yet)

Lets say i send a mail to me@???, this
I would want to hit my "first" mail servers call
it server1 (I spent a long time picking that name
:-)), this then does the lookup on it local mysql
db, and decides that the mail should be dropped on
server2.

To me this seems simple (so if anyone sees a wide
hole in the above , please fill it in), now the
problem that I forsee is when the user wishes to
retrieve the mail, is there a pop server which
would authenticate against server1 mysql db, look
for the location and get the mail there, so
basically I have no user accounts on any of the
machines just in the one db. This may be a little
off topic, but I guess itsall part and parcel
(forgive the pun) of mail delivery

Iqbal