[Exim] Doing it the larger way...some thoughts

Dear folks,

unfortunately I am not very experienced with larger scaled mail-concepts
but I currently have an idea and now I am wondering if this is
completely bullshit or perhaps a really possiblity.

Imagine there is a location with only one machine (Linux, PIII 650 with
256 MB Ram) handling some thousand users mailboxes with POP3. It is
used for POP3 and SMTP.

In case the machine gets too slow we could get a bigger and faster
server but that's not the solution for scaling larger environments.
What about several machines (e.g. 5) handling POP3/SMTP for
users (internal usernumbers like 00000001, doesn't matter).
No user would ever talk to them directly.

Additionally there are some "proxy" POP3-Servers in front of them
that get the queries from the customer's mailclients. The POP3-Proxy
looks the given User-ID up in a database and gets the information
which internal POP3-Server and with what User-ID is in charge
of that user's account. The POP3-Proxy connects to the internal
machine, logs in with the internal ID and then we just forward
the commands through the "proxy".

For incoming mail there would be several inbound mailservers
that look in a database to see to which POP3-server and to which
User-ID the mail has to be redirected.

What do you people that are more experienced in large scaled
mail-systems think of that? In case there are not too much good
arguments against this, I'd like to get vi and gcc ready to start
writing the Pop3-"Proxy" :-)

Thanks for your opinions!

Regards,
Sascha