[exim] 30,000+ email accounts

I've searched around and haven't come up with anything definative and
was hoping someone here could just point me in the right direction, I'm
sure this has come up in the past, but my searches aren't coming up with
what I'm looking for.

It appears since there are so many accounts it takes the server a long
time to parse the directory entries to get to the correct account when
someone is (a) receiving email or (b) reading their email. This equals
more load on the server and over time I'm sure we're going to reach a
load limit and lose email. We'd like to implement a solution where the
accounts are split across multiple machines, but how to split (a) the
incoming email to it's correct backend email server and (b) the
pop3/webmail requests to the correct backend servers. I know I can do
(b) webmail by using a reverse proxy, but that still wouldn't handle
pop3 or the delivery to the correct server (part a).

Any ideas?

Thanks!